ARCH


Line plots across dimensions of each concept
Line plots across concepts
Tables

Table of contents



us-gaap:CommonStockValue

us-gaap:CommonStockSharesIssued

us-gaap:WeightedAverageNumberOfDilutedSharesOutstanding

us-gaap:WeightedAverageNumberOfSharesOutstandingBasic

us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecurities

us-gaap:EarningsPerShareBasic

us-gaap:EarningsPerShareDiluted

us-gaap:ComprehensiveIncomeNetOfTax
us-gaap:StatementEquityComponents

us-gaap:CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization

us-gaap:DisposalGroupNotDiscontinuedOperationGainLossOnDisposal
us-gaap:DisposalGroupClassification, us-gaap:IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperations
us-gaap:IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperations

us-gaap:GrossProfit

us-gaap:IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:IncomeTaxExpenseBenefit

us-gaap:InterestExpense
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents
us-gaap:DebtInstrument
us-gaap:InterestExpenseDebt

us-gaap:InterestIncomeExpenseNonoperatingNet

us-gaap:NetIncomeLoss
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:NonoperatingIncomeExpense

us-gaap:OperatingIncomeLoss

us-gaap:ProfitLoss

us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax
us-gaap:ConsolidationItems
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments
us-gaap:ConsolidationItems, us-gaap:StatementGeographical
us-gaap:ConsolidationItems, us-gaap:StatementGeographical, us-gaap:StatementBusinessSegments
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents
us-gaap:StatementGeographical

us-gaap:Revenues

us-gaap:SellingGeneralAndAdministrativeExpense

us-gaap:Assets
us-gaap:ConsolidationItems
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:Liabilities

us-gaap:LiabilitiesAndStockholdersEquity

us-gaap:StockholdersEquity
us-gaap:StatementEquityComponents

us-gaap:AssetsCurrent

us-gaap:CashAndCashEquivalentsAtCarryingValue

us-gaap:CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents

us-gaap:AccountsReceivableNetCurrent
us-gaap:ConcentrationRiskByType
us-gaap:ConcentrationRiskByType
us-gaap:ReceivablesNetCurrent

us-gaap:InventoryNet

us-gaap:OtherAssetsCurrent

us-gaap:PropertyPlantAndEquipmentGross

us-gaap:AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment

us-gaap:PropertyPlantAndEquipmentNet

us-gaap:DeferredTaxAssetsLiabilitiesNet

us-gaap:OperatingLeaseRightOfUseAsset

us-gaap:OtherAssetsNoncurrent

us-gaap:AccountsPayableCurrent

us-gaap:AccruedLiabilitiesCurrent

us-gaap:DebtCurrent

us-gaap:LiabilitiesCurrent

us-gaap:OperatingLeaseLiabilityCurrent

us-gaap:OtherAccruedLiabilitiesCurrent

us-gaap:LongTermDebt

us-gaap:LongTermDebtNoncurrent
us-gaap:RetirementPlanType
us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent

us-gaap:OtherLiabilitiesNoncurrent

us-gaap:OperatingLeaseLiabilityNoncurrent

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearTwo

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearThree

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearFour

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearFive

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive

us-gaap:OperatingLeaseLiability

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDue

us-gaap:LesseeOperatingLeaseLiabilityUndiscountedExcessAmount

us-gaap:StockholdersEquity
us-gaap:StatementEquityComponents

us-gaap:CommonStockValue

us-gaap:AdditionalPaidInCapitalCommonStock

us-gaap:RetainedEarningsAccumulatedDeficit

us-gaap:AccumulatedOtherComprehensiveIncomeLossNetOfTax

us-gaap:TreasuryStockValue

us-gaap:AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue
us-gaap:StatementEquityComponents

us-gaap:NetCashProvidedByUsedInOperatingActivities

us-gaap:NetCashProvidedByUsedInInvestingActivities

us-gaap:NetCashProvidedByUsedInFinancingActivities

us-gaap:CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect

us-gaap:DepreciationDepletionAndAmortization
us-gaap:ConsolidationItems
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:IncreaseDecreaseInInventories

us-gaap:IncreaseDecreaseInOtherOperatingCapitalNet

us-gaap:NetCashProvidedByUsedInOperatingActivities

us-gaap:NetIncomeLoss
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:ProfitLoss

us-gaap:ShareBasedCompensation

us-gaap:NetCashProvidedByUsedInInvestingActivities

us-gaap:PaymentsToAcquirePropertyPlantAndEquipment
us-gaap:ConsolidationItems
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:NetCashProvidedByUsedInFinancingActivities
us-gaap:DebtInstrument
us-gaap:DebtInstrumentInterestRateStatedPercentage
us-gaap:DebtInstrument, us-gaap:SubsequentEventType

us-gaap:DeferredTaxAssetsNet

us-gaap:UnrecognizedTaxBenefits
us-gaap:StatementScenario

us-gaap:DeferredTaxAssetsValuationAllowance

aci:AdjustedEBITDA
us-gaap:ConsolidationItems
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments
us-gaap:Range, us-gaap:ConcentrationRiskByType, us-gaap:HedgingDesignation
aci:AnnualPurchaseOfCommodityForOperations
us-gaap:ScheduleOfEquityMethodInvestmentEquityMethodInvesteeName
aci:AssetImpairmentChargesAndMineClosureCosts
us-gaap:StatementBusinessSegments, us-gaap:Subsegments
us-gaap:Subsegments
us-gaap:DebtInstrument
aci:BaseCappedCallTransactionCapPrice
us-gaap:DebtInstrument
aci:BaseCappedCallTransactionInitialStrikePrice
us-gaap:DebtInstrument
aci:BaseCappedCallTransactionNumberOfUnderlyingShares
us-gaap:DebtInstrument
aci:BaseCappedCallTransactionPremiumPercentage

aci:ChangeInFairValueOfCoalDerivativesAndCoalTradingActivitiesNet
us-gaap:Range, us-gaap:DebtInstrument
aci:DebtInstrumentCovenantCrossEventsOfDefaultToIndebtedness

aci:DividendEquivalentsEarnedEquityInstrumentsOtherThanOptionsGrants
us-gaap:StatementEquityComponents

aci:EBITDFromIdledOrOtherwiseDisposedOperations

aci:EffectiveIncomeTaxRateReconciliationAmtRefundAmount
us-gaap:Ownership
aci:EquityMethodInvestmentPercentOfOwnershipInterestAcquired

aci:FinanceLeaseCost

aci:ImpairmentOfCoalLandsAndMineralRights

aci:ImpairmentOfDeferredDevelopmentCosts

aci:IncomeTaxRefundReservePercentageOfTotalRefundSubjectToAudit
us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType
aci:LineOfCreditLiquidityCovenantAmount
us-gaap:DebtInstrument

aci:OperatingCostExpensesAndOthers

aci:OperatingIncomeExpenseRelatedToJointventure

aci:OtherIncomeExpense

aci:ProceedsFromConsiderationPaidForDisposalsAndDivestitures

aci:ProceedsFromEquipmentFinancing

aci:PurchasesOfInvestmentsAdvancesToAffiliates
us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation
aci:RealizedAndUnrealizedGainLossOnDerivatives

aci:SegmentAdjustedEBITDFromCoalOperations
aci:DefinedBenefitPlansBenefitObligationExpenses
aci:TotalWorkersCompensationExpense

aci:ValueOfTradingPortfolioRealizedGainLoss

us-gaap:AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts
us-gaap:DebtInstrument
us-gaap:AmortizationOfDebtDiscountPremium

us-gaap:AmortizationOfFinancingCostsAndDiscounts

us-gaap:AssetImpairmentCharges
us-gaap:StatementBusinessSegments, us-gaap:Subsegments

us-gaap:AssetRetirementObligationAccretionExpense
us-gaap:ConsolidationItems
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:BankruptcyClaimsAmountOfClaimsFiled

us-gaap:DebtConversionOriginalDebtAmount1
us-gaap:DebtInstrument
us-gaap:DebtInstrumentConvertibleConversionRatio1
us-gaap:DebtInstrument
us-gaap:DebtInstrumentRedemptionPricePercentage
aci:DefinedBenefitPlansBenefitObligationExpenses
us-gaap:DefinedBenefitPlanAmortizationOfGainsLosses
us-gaap:RetirementPlanType
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents
us-gaap:DefinedBenefitPlanAmortizationOfPriorServiceCostCredit
us-gaap:RetirementPlanType
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanExpectedReturnOnPlanAssets
aci:DefinedBenefitPlansBenefitObligationExpenses
us-gaap:DefinedBenefitPlanInterestCost
us-gaap:RetirementPlanType
aci:DefinedBenefitPlansBenefitObligationExpenses
us-gaap:DefinedBenefitPlanNetPeriodicBenefitCost
us-gaap:RetirementPlanType
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents
us-gaap:DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1
us-gaap:RetirementPlanType
aci:DefinedBenefitPlansBenefitObligationExpenses
us-gaap:DefinedBenefitPlanServiceCost
us-gaap:RetirementPlanType
us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation
us-gaap:DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
aci:ReportingYear, us-gaap:Position
us-gaap:DerivativeNonmonetaryNotionalAmountMass
us-gaap:Position
us-gaap:ConcentrationRiskByType, us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation
us-gaap:DerivativeNonmonetaryNotionalAmountVolume
us-gaap:HedgingDesignation, us-gaap:TradingActivityByType

us-gaap:DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax

us-gaap:EquityMethodInvestmentOtherThanTemporaryImpairment
us-gaap:ScheduleOfEquityMethodInvestmentEquityMethodInvesteeName
us-gaap:FairValueByFairValueHierarchyLevel
us-gaap:FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss

us-gaap:FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1

us-gaap:FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues

us-gaap:FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases

us-gaap:FinanceLeaseInterestPaymentOnLiability
us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation
us-gaap:GainLossOnSaleOfDerivatives

us-gaap:GainLossRelatedToLitigationSettlement
us-gaap:GainContingenciesByNature

us-gaap:GainOnBusinessInterruptionInsuranceRecovery
us-gaap:BusinessInterruptionLoss

us-gaap:ImpairmentOfLongLivedAssetsHeldForUse

us-gaap:IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance

us-gaap:IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate

us-gaap:IncomeTaxReconciliationNondeductibleExpense

us-gaap:IncomeTaxReconciliationNondeductibleExpenseDepletion

us-gaap:IncomeTaxReconciliationOtherAdjustments

us-gaap:IncomeTaxReconciliationStateAndLocalIncomeTaxes

us-gaap:IncomeTaxReconciliationTaxSettlements

us-gaap:IncreaseDecreaseInDeferredIncomeTaxes

us-gaap:IncreaseDecreaseInDerivativeAssetsAndLiabilities

us-gaap:IncreaseDecreaseInIncomeTaxesPayableNetOfIncomeTaxesReceivable

us-gaap:IncreaseDecreaseInOtherAccountsPayableAndAccruedLiabilities

us-gaap:IncreaseDecreaseInReceivables
us-gaap:DebtInstrument
us-gaap:InterestExpenseDebtExcludingAmortization
us-gaap:DerivativeInstrumentRisk
us-gaap:InterestRateCashFlowHedgeGainLossReclassifiedToEarningsNet

us-gaap:NondebtorReorganizationItemsNetGainLossOnSettlementOfOtherClaims1

us-gaap:OperatingLeaseCost

us-gaap:OperatingLeasePayments

us-gaap:OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentBeforeTaxPortionAttributableToParent

us-gaap:OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent

us-gaap:OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentBeforeTaxPortionAttributableToParent

us-gaap:OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent

us-gaap:OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax
us-gaap:StatementEquityComponents

us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent

us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationBeforeTaxParent
us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation, us-gaap:Position
us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax
us-gaap:HedgingDesignation

us-gaap:OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent
aci:DefinedBenefitPlansBenefitObligationExpenses
us-gaap:OtherLaborRelatedExpenses

us-gaap:OtherNonoperatingIncome
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:OtherOperatingIncomeExpenseNet

us-gaap:PaymentsOfFinancingCosts

us-gaap:PaymentsOfOrdinaryDividends

us-gaap:PaymentsRelatedToTaxWithholdingForShareBasedCompensation

us-gaap:PaymentsToAcquireAvailableForSaleSecuritiesDebt

us-gaap:PaymentsToAcquireRoyaltyInterestsInMiningProperties

us-gaap:PensionAndOtherPostretirementBenefitExpense
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents
us-gaap:PostemploymentBenefitsPeriodExpense

us-gaap:ProceedsFromConvertibleDebt
us-gaap:DebtInstrument

us-gaap:ProceedsFromInsuranceSettlementInvestingActivities

us-gaap:ProceedsFromIssuanceOfDebt
us-gaap:DebtInstrument
us-gaap:ProceedsFromIssuanceOfSecuredDebt

us-gaap:ProceedsFromRepaymentsOfOtherDebt

us-gaap:ProceedsFromRepaymentsOfOtherLongTermDebt

us-gaap:ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities
us-gaap:ScheduleOfEquityMethodInvestmentEquityMethodInvesteeName
us-gaap:ProfessionalFees
us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation, us-gaap:Position
us-gaap:ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax
us-gaap:HedgingDesignation

us-gaap:ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax
us-gaap:StatementEquityComponents

us-gaap:SeveranceCosts1

us-gaap:StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures

us-gaap:StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures
us-gaap:StatementEquityComponents

us-gaap:StockRepurchasedAndRetiredDuringPeriodValue
us-gaap:StatementEquityComponents
us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation
us-gaap:UnrealizedGainLossOnDerivatives

us-gaap:WeightedAverageNumberDilutedSharesOutstandingAdjustment

aci:CoalLandsAndMineralRights

aci:DeferredMineDevelopment

aci:DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTaxAndOtherTaxCredits
us-gaap:RetirementPlanType
aci:DefinedBenefitPlanExpectedFutureBenefitPaymentsTotal

aci:DefinedBenefitPlanFairValueOfPlanAssetsNet

aci:DerivativeAssetExcludingInterestRateSwaps

aci:IncomeTaxRefundReserve

aci:LeaseAssets

aci:LeaseLiabilities
us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType
aci:LineOfCreditFacilityCovenantComplianceLiquidityAmount

aci:OperatingLossCarryforwardsAndTaxCreditCarryforwardsLimitationsOnUseAnnualAmount

aci:OtherAssetsNonCurrentTotal
us-gaap:ConcentrationRiskByType
aci:PercentageOfAccountsReceivableNetCurrent

aci:RevenueRemainingPerformanceObligationFixedPriceVolume
us-gaap:ProductOrService
us-gaap:ProductOrService
aci:RevenueRemainingPerformanceObligationMass

aci:RevenueRemainingPerformanceObligationVariablePriceVolume
us-gaap:ProductOrService

aci:SuretyBondsOutstanding
us-gaap:RegulatoryLiability

us-gaap:AccrualForTaxesOtherThanIncomeTaxesCurrent

us-gaap:AllowanceForDoubtfulAccountsReceivableCurrent

us-gaap:AssetRetirementObligation

us-gaap:AssetRetirementObligationCurrent

us-gaap:AssetRetirementObligationsNoncurrent

us-gaap:AssetsFairValueDisclosure
us-gaap:FairValueByFairValueHierarchyLevel

us-gaap:AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
us-gaap:FinancialInstrument

us-gaap:AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
us-gaap:FinancialInstrument

us-gaap:AvailableForSaleDebtSecuritiesAmortizedCostBasis
us-gaap:FinancialInstrument

us-gaap:AvailableForSaleSecuritiesDebtSecurities
us-gaap:FairValueByFairValueHierarchyLevel
us-gaap:FinancialInstrument

us-gaap:AvailableForSaleSecuritiesDebtSecuritiesCurrent
us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LongtermDebtType
us-gaap:CashCollateralForBorrowedSecurities
us-gaap:DebtInstrument
us-gaap:ConvertibleDebtCurrent

us-gaap:DebtInstrumentCarryingAmount
us-gaap:DebtInstrument
us-gaap:DebtInstrument
us-gaap:DebtInstrumentConvertibleConversionPrice1
us-gaap:DebtInstrument
us-gaap:DebtInstrumentFaceAmount
us-gaap:DebtInstrument, us-gaap:SubsequentEventType

us-gaap:DebtInstrumentFairValue
aci:DefinedBenefitPlansBenefitObligationExpenses, us-gaap:MeasurementInputType
us-gaap:DebtInstrumentMeasurementInput
us-gaap:MeasurementInputType

us-gaap:DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger

us-gaap:DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months

us-gaap:DeferredFinanceCostsCurrentNet

us-gaap:DeferredFinanceCostsGross

us-gaap:DeferredFinanceCostsNet
us-gaap:DebtInstrument

us-gaap:DeferredIncomeTaxLiabilities
us-gaap:DebtInstrument
us-gaap:DeferredOfferingCosts

us-gaap:DeferredTaxAssetsGross

us-gaap:DeferredTaxAssetsInvestmentInSubsidiaries

us-gaap:DeferredTaxAssetsOperatingLossCarryforwards

us-gaap:DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration

us-gaap:DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAssetRetirementObligations

us-gaap:DeferredTaxLiabilitiesFinancingArrangements

us-gaap:DeferredTaxLiabilitiesOtherFiniteLivedAssets

us-gaap:DeferredTaxLiabilitiesPropertyPlantAndEquipment

us-gaap:DefinedBenefitPensionPlanLiabilitiesNoncurrent
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanAccumulatedBenefitObligation
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax
us-gaap:RetirementPlanName, us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
us-gaap:RetirementPlanType
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanBenefitObligation
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
us-gaap:RetirementPlanType
us-gaap:DefinedBenefitPlanFundedStatusOfPlan

us-gaap:DefinedBenefitPlanHealthCareCostTrendRateAssumedNextFiscalYear

us-gaap:DefinedBenefitPlanUltimateHealthCareCostTrendRate1

us-gaap:DepositsAssetsNoncurrent

us-gaap:DerivativeAssetCollateralObligationToReturnCashOffset

us-gaap:DerivativeAssetFairValueGrossLiability

us-gaap:DerivativeAssets

us-gaap:DerivativeAssetsLiabilitiesAtFairValueNet
us-gaap:BalanceSheetLocation
us-gaap:BalanceSheetLocation, us-gaap:DerivativeInstrumentRisk
us-gaap:ConcentrationRiskByType, us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation
us-gaap:DerivativeAveragePriceRiskOptionStrikePrice
us-gaap:HedgingDesignation, us-gaap:TradingActivityByType

us-gaap:DerivativeCollateralObligationToReturnCash

us-gaap:DerivativeFairValueOfDerivativeAsset
us-gaap:DerivativeInstrumentRisk
us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation
us-gaap:HedgingDesignation
us-gaap:HedgingDesignation, us-gaap:TradingActivityByType

us-gaap:DerivativeFairValueOfDerivativeLiability
us-gaap:DerivativeInstrumentRisk
us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation
us-gaap:HedgingDesignation
us-gaap:HedgingDesignation, us-gaap:TradingActivityByType

us-gaap:DerivativeLiabilities

us-gaap:DerivativeLiabilityCollateralRightToReclaimCashOffset
us-gaap:BalanceSheetLocation

us-gaap:DerivativeLiabilityFairValueGrossAsset

us-gaap:EmployeeRelatedLiabilitiesCurrent
us-gaap:AwardType
us-gaap:EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized

us-gaap:EnergyRelatedInventoryCoal
us-gaap:Ownership
us-gaap:EquityMethodInvestmentOwnershipPercentage

us-gaap:EquityMethodInvestments
us-gaap:ScheduleOfEquityMethodInvestmentEquityMethodInvesteeName

us-gaap:FinanceLeaseLiability

us-gaap:FinanceLeaseLiabilityCurrent

us-gaap:FinanceLeaseLiabilityNoncurrent

us-gaap:FinanceLeaseLiabilityPaymentsDue

us-gaap:FinanceLeaseLiabilityPaymentsDueYearFive

us-gaap:FinanceLeaseLiabilityPaymentsDueYearFour

us-gaap:FinanceLeaseLiabilityPaymentsRemainderOfFiscalYear

us-gaap:FinanceLeaseLiabilityUndiscountedExcessAmount

us-gaap:FinanceLeaseRightOfUseAsset

us-gaap:InsuranceSettlementsReceivableCurrent

us-gaap:InsuranceSettlementsReceivableNoncurrent

us-gaap:InterestPayableCurrent

us-gaap:InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet

us-gaap:InterestRateDerivativeAssetsAtFairValue
us-gaap:FairValueByFairValueHierarchyLevel

us-gaap:InterestRateDerivativeLiabilitiesAtFairValue
us-gaap:FairValueByFairValueHierarchyLevel

us-gaap:InventoryPartsAndComponentsNetOfReserves

us-gaap:InventoryValuationReserves

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths

us-gaap:LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear

us-gaap:LettersOfCreditOutstandingAmount
us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType
us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LongtermDebtType
us-gaap:CreditFacility, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType
us-gaap:CreditFacility, us-gaap:LongtermDebtType

us-gaap:LiabilityForFuturePolicyBenefitsDisabilityAndAccidentBenefits
us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType
us-gaap:LineOfCreditFacilityCurrentBorrowingCapacity
us-gaap:CreditFacility, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType

us-gaap:LitigationReserve

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo

us-gaap:MineReclamationAndClosingLiabilityNoncurrent

us-gaap:MineralRights
us-gaap:GainContingenciesByNature
us-gaap:NontradeReceivablesCurrent

us-gaap:OperatingLeaseWeightedAverageDiscountRatePercent

us-gaap:OperatingLossCarryforwards

us-gaap:OtherPostretirementDefinedBenefitPlanLiabilitiesNoncurrent

us-gaap:OtherReceivables
us-gaap:RetirementPlanType
us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities
us-gaap:RetirementPlanType
us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent

us-gaap:PropertyPlantAndEquipmentOther

us-gaap:RestrictedCashCurrent
us-gaap:DebtInstrument
aci:DefinedBenefitPlansBenefitObligationExpenses
us-gaap:SelfInsuranceReserveNoncurrent
us-gaap:AwardType
us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
us-gaap:AwardType
us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
us-gaap:PlanName
us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
us-gaap:PlanName
us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant

us-gaap:StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1

us-gaap:TreasuryStockCommonShares

us-gaap:TreasuryStockShares

us-gaap:UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued

us-gaap:UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary
us-gaap:ValuationAllowancesAndReservesType
us-gaap:ValuationAllowancesAndReservesBalance

us-gaap:WorkersCompensationLiabilityCurrent

us-gaap:WorkersCompensationLiabilityCurrentAndNoncurrent

us-gaap:WorkersCompensationLiabilityNoncurrent

Plots across concepts


IncomeStatement
Net Income Loss
Expenses
Income Tax Expense Benefit
Balance Sheet
Operating Lease Right Of Use Asset
Assets
Derivative Assets
Current Assets
Other Assets Current
NonCurrent Assets
Operating Lease Right Of Use Asset
PropertyPlantAndEquipmentNet
Accumulated Depreciation Depletion And Amortization Property Plant And Equipment
Liabilities
Derivative Liabilities
Current Liabilities
Operating Lease Liability Current
Noncurrent Liabilities
Operating Lease Liability Noncurrent
Stockholders Equity
Accumulated Other Comprehensive Income Loss Net Of Tax
CashFlow
Net Cash Provided By Used In Investing Activities
NetCashProvidedByUsedInOperatingActivities
Amortization Of Financing Costs And Discounts
NetCashProvidedByUsedInInvestingActivities
Net Cash Provided By Used In Investing Activities
NetCashProvidedByUsedInFinancingActivities
Net Cash Provided By Used In Financing Activities
Shares
Treasury Stock Shares
OperatingLeaseLiability
Operating Lease Liability Current
NonoperatingIncomeExpense
Nonoperating Income Expense
LesseeOperatingLeaseLiabilityPaymentsDue
Lessee Operating Lease Liability Payments Remainder Of Fiscal Year
AccountsReceivable
Allowance For Doubtful Accounts Receivable Current

Tables

  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:CommonStockValue ██▆▄▄▄▄▁▁▁▁▁▁▁▁▁▁▁▁0.2540.2540.2530.2520.2520.2520.2520.250.250.250.250.250.250.250.250.250.250.250.25
us-gaap:CommonStockSharesIssued ██▇▅▅▅▅▂▂▂▂▂▂▂▂▂▁▁▁25.38225.38225.32325.23525.23525.23125.2225.0525.04725.04725.04725.04725.04725.04725.04725.03825.02125.02125.002
us-gaap:WeightedAverageNumberOfDilutedSharesOutstanding █▁16.75615.283NA15.14715.14515.139NA16.85217.78118.599NA20.20821.03621.875NA24.13525.08225.40825.469
us-gaap:WeightedAverageNumberOfSharesOutstandingBasic █▁15.29415.283NA15.14715.14515.139NA15.73616.54317.494NA19.2520.15620.901NA23.5824.65925.00825.002
us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount NA1.158NANANA0.236575NANANANANANANANANANANANANA
 us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecurities us-gaap:WarrantNANANA0.178333NA0.114775NANANANANANANANANANANANANA
us-gaap:EarningsPerShareBasic ▆▅▄▁▄▅▅█▇▇▇█▆▇▇▇▆▆▆1.82-0.4-5.169999999999998-12.64-3.26-1.67-0.18999999999999956.793.84.164.6300000000000016.42.152.873.61000000000000032.91.512.071.34
us-gaap:EarningsPerShareDiluted ▆▆▄▁▄▅▅█▇▇▇█▆▇▇▇▆▆▆1.66-0.4-5.169999999999998-12.64-3.26-1.67-0.576.343.533.914.446.12.062.743.642.831.482.031.31
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
 TTM Greenblatt ROC _ _▆▅▄▁▄▅▅█▇▇█▇▆▇▇▇▆▇▇30.411-4.137-77.518-190.21-47.016-26.02-6.96106.14164.28374.4986.4979.50841.09761.04551.11768.05837.76358.08746.118
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:ComprehensiveIncomeNetOfTax ▆▅▃▁▄▄▄▇▇▇██▆▇▇▇▆▆▆32.45-4.705-105.457-187.082-50.764-42.702-43.173100.29263.23875.835108.685126.09434.84365.88482.68159.81140.57151.357.903
us-gaap:CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization ▃▁▂▂▁▃▇▇▆▅█▇▇355.329309.906341.593345.539316.348374.999492.454491.004451.088438.471514.005482.029474.388NANANANANANA
us-gaap:DisposalGroupNotDiscontinuedOperationGainLossOnDisposal NANA0.1360.01.369NA-9.0080.0-4.304NA0.00.00.0NA-0.27721.574NANA0.0
 us-gaap:DisposalGroupNotDiscontinuedOperationGainLossOnDisposal us-gaap:DisposalGroupClassification,us-gaap:IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperations us-gaap:DiscontinuedOperationsDisposedOfBySale,aci:ViperMineNANA0.1NANANANANANANANANANANANANANANANA
 us-gaap:DisposalGroupNotDiscontinuedOperationGainLossOnDisposal us-gaap:IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperations aci:DalTexAndBriarBranchPropertiesNANA0.1360.01.369NA-9.0080.0-4.304NANANANANANANANANANA
us-gaap:GrossProfit ▆▅▄▁▄▅▆█▇▇█▇▇▇▇▇▆▇33.205-0.337-73.283-187.68-45.286-23.89123.144106.48166.5776.779101.39682.88644.59565.16763.4673.3142.92466.985NA
us-gaap:IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest ▆▅▄▁▄▅▅█▇▇▇▇▆▇▇▇▆▇▆29.872-5.664-78.326-191.088-48.118-27.09-8.811107.11662.93172.81182.74377.97739.9459.44146.566.70837.47952.50834.605
us-gaap:IncomeTaxExpenseBenefit █████▇████▇▁▇█▃▇███2.0060.3780.1990.3791.206-1.791-0.260.3470.0910.07-3.351-45.2-3.366-0.544-34.771-1.6430.3190.841.156
us-gaap:InterestExpense ▁▂▂▁▁▁▂▂▂▂▃▃▃▃▃▄▄▆█2.9414.1284.5322.9893.5233.3883.6294.0494.3754.4324.8475.1795.055.3955.5055.9726.0039.42511.241
 us-gaap:InterestExpenseDebt us-gaap:DebtInstrument aci:ConvertibleSeniorNotesDue2025██▁3.83.72.3NANANANANANANANANANANANANANANANA
us-gaap:InterestIncomeExpenseNonoperatingNet ▆▆▅▇▇▇▇█▇▇▆▆▆▅▅▅▅▂▁-2.794-3.8-4.235-2.53-1.73-2.129-1.878-0.34-2.287-2.289-2.691-3.378-3.498-4.122-4.945-5.252-5.161-8.898-10.754
us-gaap:NetIncomeLoss ▆▅▄▁▄▅▅█▇▇▇█▆▇▇▇▆▆▆27.866-6.042-78.525-191.467-49.324-25.299-8.551106.76962.8472.74186.094123.19243.30659.98581.27168.35137.1651.66833.449
us-gaap:NonoperatingIncomeExpense ▅▅▅▅▅▅▆▇▅▄▅▅▅▅█▅▆▁▅-0.539-1.527-0.808-0.878-1.102-1.070.0270.975-1.352-1.679-1.056-1.531-1.157-1.6042.268-1.35-0.284-5.579-0.759
us-gaap:OperatingIncomeLoss ▆▅▄▁▄▅▅█▇▇█▇▆▇▇▇▆▇▇30.411-4.137-77.518-190.21-47.016-26.02-6.96106.14164.28374.4986.4979.50841.09761.04551.11768.05837.76358.08746.118
us-gaap:ProfitLoss █▁27.866-6.042NANA-49.324-25.299NANANANANANANANANANANANA33.449
us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax ▄▂▂▂▁▃▆▇▆▆██▇▆▆▇▆▇▆450.389357.543360.578382.261319.521405.232549.48619.467570.222555.183650.963633.18592.349575.295560.244613.538549.866600.975575.688
us-gaap:Revenues ▄▂▂▂▁▃▆▇▆▆██▇▆▆▇▆▇▆450.389357.543360.578382.261319.521405.232549.48619.467570.222555.183650.963633.18592.349575.295560.244613.538549.866600.975575.688
us-gaap:SellingGeneralAndAdministrativeExpense ▆▄▁▄▂▅▄▆▇▆█▅▆▇▄▃▄▃▅24.11921.4818.37321.54119.73822.74521.91724.56625.20924.08926.68722.90924.75625.94822.31321.2922.45620.76222.836
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:Assets ▃▃▂▁▃▄▄▅▄▄▄▆▅▆▆▆███1794.9461774.1581722.4721653.3371802.3891857.5061867.7561934.61889.531863.2011887.061997.6221949.4131976.6781979.6322022.5332132.0582144.9162136.597
us-gaap:Liabilities ██▇▄▃▃▂▂▂▁▁▄▄▄▄▅▅▅▆1476.4171492.7151438.9111287.6911254.6071263.7421227.221229.1641191.581163.2541182.2391310.4821308.4651288.2161313.7671350.0541347.01343.211390.02
us-gaap:LiabilitiesAndStockholdersEquity ▃▃▂▁▃▄▄▅▄▄▄▆▅▆▆▆███1794.9461774.1581722.4721653.3371802.3891857.5061867.7561934.61889.531863.2011887.061997.6221949.4131976.6781979.6322022.5332132.0582144.9162136.597
us-gaap:StockholdersEquity ▁▁▁▂▅▅▆▇▇▇▇▆▆▆▆▆██▇318.529281.443283.561365.646547.782593.764640.536705.436697.95699.947704.821687.14640.948688.462665.865672.479785.058801.706746.577
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:AssetsCurrent ▁▂▂▁▁▃▄▇▆▆▇▇▆▇▆▇█▇▇555.046583.2588.14562.982564.502639.537706.747866.969840.693832.836878.783877.417857.305867.705830.994869.619945.415911.207878.622
us-gaap:CashAndCashEquivalentsAtCarryingValue ▂▃▃▂▂▁▂▃▅▄▆▅▅▆▆▇██▇153.516169.593187.492156.655150.022105.157153.02175.428232.429218.75264.937245.679241.59288.332273.387298.337333.548347.58305.372
us-gaap:CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents ▂▃▃▂▂▁▂▃▄▄▅▄▄▅▅▅▇█▇160.62188.555193.445170.574150.022105.157153.02175.428232.429218.75264.937245.679241.59291.258273.602298.339375.303416.564376.422
us-gaap:AccountsReceivableNetCurrent ▅▂▁▂▂▄▅█▆▇▇▆▇█▅█▇▄▆160.154129.086110.869127.292118.835149.225168.125206.149182.824195.095200.904183.318198.362208.1172.604207.059190.256150.399184.483
 us-gaap:ReceivablesNetCurrent us-gaap:ConcentrationRiskByType aci:ElectricUtilitiesNANA41.7NANANA69.1NANANA73.9NANANA72.9NANANA96.0
us-gaap:InventoryNet ▇▆▂▅▆▅▃██▅▂▇▆▅▃▂▄▂▁162.531154.395126.008143.396154.69149.986130.898171.543172.841145.607125.47163.878157.205143.831128.96122.436137.056125.194113.462
us-gaap:OtherAssetsCurrent ▁▁▃▃▄▆▇█▃▄▅▆▆▄▄▄▃▄▆35.14939.91758.055.71270.75887.09697.894113.50256.99364.19275.74995.0486.64268.45970.42663.68561.47474.06396.306
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:PropertyPlantAndEquipmentGross NANA1428.982NANANA1342.046NANANA1101.171NANANA1103.39NANANA1086.096
us-gaap:AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment NANA421.679NANANA357.537NANANA266.343NANANA147.442NANANA32.493
us-gaap:PropertyPlantAndEquipmentNet █▇▆▄▇▆▅▂▂▁▁▃▃▄▄▅▆▆▇1102.571058.9421007.303964.4721066.6141035.776984.509889.295870.889848.749834.828919.613925.559935.511955.948980.8011007.571027.2881053.603
us-gaap:DeferredTaxAssetsLiabilitiesNet NANA0.037NANANA0.08NANANA0.17NANANA22.519NANANA0.113
us-gaap:OperatingLeaseRightOfUseAsset ▁▄▆█15.74416.46517.117.6NANANANANANANANANANANANANANANA
us-gaap:OtherAssetsNoncurrent ▂▁▁▁▂▄▃▃▃▄▃▆▂▃▂▂▄▇█62.35657.51355.24655.60865.14875.65869.2370.79371.87676.19768.00395.26762.3668.67659.78365.93475.058102.277108.298
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:AccountsPayableCurrent ▃▄▂▁▂▂▅█▅▅▄▃▄▄▅▅▄▂▁116.042122.916103.74399.11104.464108.972133.06166.129138.735138.935128.024119.629131.027127.149134.137137.664127.059103.1395.953
us-gaap:AccruedLiabilitiesCurrent ▄▂▃▁▁▁▃▃▃▁▆█▇▃▆▄▄▄█166.287150.167155.256141.021143.304143.054157.167161.939162.741143.586183.514203.667194.73158.882184.161164.939169.533172.898205.24
us-gaap:DebtCurrent █▂▂▂▂▂▂▁▁▁▂▁▁▁▁▁▁▁▁133.81224.59731.09725.98725.70228.45620.75311.92513.06815.2117.79711.47812.53314.32815.7838.5667.4147.89811.038
us-gaap:LiabilitiesCurrent █▂▂▁▁▂▃▄▃▂▄▄▄▃▄▃▃▂▃416.141297.68290.096266.118273.47280.482310.98339.993314.544297.731329.335334.774338.29300.359334.081311.169304.006283.926312.231
us-gaap:OperatingLeaseLiabilityCurrent █▇▄▁2.4862.4762.4542.427NANANANANANANANANANANANANANANA
us-gaap:OtherAccruedLiabilitiesCurrent ▅▃▃▂▂▂▂▂▂▁▄▁█▂▃▂▁▁▁21.43815.25214.4959.81211.24710.42610.5599.9519.8766.03817.4485.6334.268.73614.1939.0465.3016.4126.194
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:LongTermDebt ██▆▁535.888543.954508.312394.265NANA310.819NANANANANANANANANANANANA
us-gaap:LongTermDebtNoncurrent ▄█▇▃▂▂▁▁▁▁▁▁▁▂▂▂▂▂▃402.076519.357477.215368.278323.854330.036290.066292.781295.263297.733300.186302.83305.157307.742310.134312.604315.639318.03351.841
 us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent us-gaap:RetirementPlanType us-gaap:OtherPostretirementBenefitPlansDefinedBenefitNANA94.388NANANA80.567NANANA83.163NANANA102.369NANANANA
 us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent us-gaap:RetirementPlanType us-gaap:PensionPlansDefinedBenefitNANA2.879NANANA5.476NANANA16.147NANANA14.036NANANANA
us-gaap:OtherLiabilitiesNoncurrent ▇█▇█▇▇▅▇▅▄▁▃▁▃▂▅▃▅▃97.792103.90698.906102.0898.65895.66382.193.03380.19468.57748.80161.89649.19464.1459.45776.92365.04877.30163.824
us-gaap:OperatingLeaseLiabilityNoncurrent ▁▄▆█13.94214.63615.27815.878NANANANANANANANANANANANANANANA
us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearTwo ▁▁██3.2853.2853.3173.317NANANANANANANANANANANANANANANA
us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearThree ▁▁██3.23.23.2853.285NANANANANANANANANANANANANANANA
us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearFour ▁▁██3.1853.1853.23.2NANANANANANANANANANANANANANANA
us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearFive NANA3.1857.798NANANANANANANANANANANANANANANA
us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive NANA4.614NANANANANANANANANANANANANANANANA
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:StockholdersEquity ▁▁▁▂▅▅▆▇▇▇▇▆▆▆▆▆██▇318.529281.443283.561365.646547.782593.764640.536705.436697.95699.947704.821687.14640.948688.462665.865672.479785.058801.706746.577
us-gaap:CommonStockValue ██▆▄▄▄▄▁▁▁▁▁▁▁▁▁▁▁▁0.2540.2540.2530.2520.2520.2520.2520.250.250.250.250.250.250.250.250.250.250.250.25
us-gaap:AdditionalPaidInCapitalCommonStock ██▇▆▅▅▄▅▄▄▃▃▃▂▂▂▂▁▁774.665770.052767.484744.112739.156734.315730.551734.829728.996723.143717.492712.295708.127703.98700.125697.324694.78692.253688.424
us-gaap:RetainedEarningsAccumulatedDeficit ▅▄▄▅▇███▇▆▆▅▄▄▃▂▂▂▁400.771372.882378.906457.432648.909698.292731.425746.928647.44592.296527.666449.122333.753298.664247.232173.676113.57485.11733.449
us-gaap:AccumulatedOtherComprehensiveIncomeLossNetOfTax ▂▁▁▃▃▃▅▇███▆▆▆▆▆▆▆▆-29.78-34.364-35.701-8.769-13.154-11.7145.68940.31146.78846.3943.29620.70517.80326.26620.36718.95727.49724.08624.454
us-gaap:TreasuryStockValue ████████▇▇▆▅▄▄▃▃▁827.381827.381827.381827.381827.381827.381827.381816.882725.524662.132583.883495.232418.985340.698302.109217.72851.043NA0.0
us-gaap:AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue █▁4.6363.903NA4.9714.8913.962NA5.8335.8225.651NA4.1684.1473.845NANANANA1.032
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:NetCashProvidedByUsedInOperatingActivities ▂▂▂▃▃▁▅▆█▅█▇▅▄▆▆▅▇▅20.4235.6865.19229.99237.957-12.03585.661108.147140.92284.984151.848121.11978.12966.86794.328101.34675.27125.52984.192
us-gaap:NetCashProvidedByUsedInInvestingActivities ▄▄▁▄▇▃▂▃▄▄▄▅▄▆█▆▄▄▇-38.006-42.765-114.064-53.65316.588-74.88-85.109-64.309-47.493-42.2-39.686-28.94-36.8071.48140.849-2.441-52.075-46.13517.984
us-gaap:NetCashProvidedByUsedInFinancingActivities ▅▆█▆▅▆▄▃▃▃▃▃▃▄▃▁▄▄▅-10.35232.189131.74344.213-9.6839.052-22.96-100.839-79.75-88.971-92.904-88.09-90.99-50.692-89.079-175.869-64.456-39.2522.709
us-gaap:CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect ▃▄▅▅▆▂▃▂▅▂▅▄▂▅▃▁▂▆█-27.935-4.8922.87120.55244.865-47.863-22.408-57.00113.679-46.18719.2584.089-49.66817.656-24.737-76.964-41.26140.14294.373
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:DepreciationDepletionAndAmortization ▃▁▃█▆▇▅▆▂▁▃▇▆▅▄▇▆▇█27.88425.79727.44732.6330.16731.30829.42230.24926.53525.27327.53631.77530.54929.70327.92831.91430.70131.92132.604
us-gaap:IncreaseDecreaseInInventories ▆█▅▄▆▇▂▅█▇▁▆▆▇▆▄▆▆▂8.13628.387-3.373-11.2934.70419.088-30.822-1.29727.23320.137-38.4086.67313.37414.8716.524-10.49211.86211.732-24.87
us-gaap:IncreaseDecreaseInOtherOperatingCapitalNet ▁▂▃▁▃▁▃▂▃▂█▃▅▂▅▃▂▂▆-17.285-8.621-3.104-19.269-2.095-16.865-0.214-13.5962.861-6.19744.962-1.22917.568-8.00518.70.945-14.593-6.96422.2
us-gaap:NetCashProvidedByUsedInOperatingActivities ▂▂▂▃▃▁▅▆█▅█▇▅▄▆▆▅▇▅20.4235.6865.19229.99237.957-12.03585.661108.147140.92284.984151.848121.11978.12966.86794.328101.34675.27125.52984.192
us-gaap:NetIncomeLoss ▆▅▄▁▄▅▅█▇▇▇█▆▇▇▇▆▆▆27.866-6.042-78.525-191.467-49.324-25.299-8.551106.76962.8472.74186.094123.19243.30659.98581.27168.35137.1651.66833.449
us-gaap:ProfitLoss █▁27.866-6.042NANA-49.324-25.299NANANANANANANANANANANANA33.449
us-gaap:ShareBasedCompensation ▆▅▅▇▇▅▆███▇▆▆▅▄▃▃▃▁4.6133.8853.5284.9714.9743.9624.6845.8325.8225.6515.3584.1694.1473.8452.9522.5432.5162.4261.032
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:NetCashProvidedByUsedInInvestingActivities ▄▄▁▄▇▃▂▃▄▄▄▅▄▆█▆▄▄▇-38.006-42.765-114.064-53.65316.588-74.88-85.109-64.309-47.493-42.2-39.686-28.94-36.8071.48140.849-2.441-52.075-46.13517.984
us-gaap:PaymentsToAcquirePropertyPlantAndEquipment ▅▅▅▄▄▆█▃▃▃▃▂▂▁▂▁▁▁▂71.19976.75880.1657.160.87287.69128.9649.45248.70839.14739.5325.69320.5969.45328.70213.58110.9725.9515.214
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
us-gaap:NetCashProvidedByUsedInFinancingActivities ▅▆█▆▅▆▄▃▃▃▃▃▃▄▃▁▄▄▅-10.35232.189131.74344.213-9.6839.052-22.96-100.839-79.75-88.971-92.904-88.09-90.99-50.692-89.079-175.869-64.456-39.2522.709
  2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
 us-gaap:DebtInstrumentInterestRateStatedPercentage us-gaap:DebtInstrument aci:ConvertibleSeniorNotesDue2025NANANANANANANANANANANANANANANANANANANA
 us-gaap:DebtInstrumentInterestRateStatedPercentage us-gaap:DebtInstrument aci:TaxExemptBondsNANANANANANANANANANANANANANANANANANANA
 us-gaap:DebtInstrumentInterestRateStatedPercentage us-gaap:DebtInstrument,us-gaap:SubsequentEventType us-gaap:Bonds,us-gaap:SubsequentEventNANANANANANANANANANANANANANANANANANANA

Common Stock Value

us-gaap:CommonStockValue

us-gaap:CommonStockValue


Common
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value254000.0254000.0253000.0252000.0252000.0252000.0252000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.02145000.02145000.02145000.02145000.02145000.02145000.02145000.02141000.02141000.02141000.02141000.02141000.02141000.02141000.02141000.02141000.02141000.02138000.02141000.02136000.02135000.02127000.01647000.01645000.01643000.0
Percentage of CommonStockValue100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Common Stock Shares Issued

us-gaap:CommonStockSharesIssued

us-gaap:CommonStockSharesIssued


Common
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value252525252525252525252525252525252525252121212121214214213213213213213213213213213213213213213212164164163
Percentage of CommonStockSharesIssued100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Weighted Average Number Of Diluted Shares Outstanding

us-gaap:WeightedAverageNumberOfDilutedSharesOutstanding

us-gaap:WeightedAverageNumberOfDilutedSharesOutstanding


Weighted
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-12-312013-09-302013-06-302013-03-312012-09-302012-06-302012-03-312011-09-302011-06-302011-03-312010-09-302010-06-302010-03-31
Value161515151516171820212124252525212212212212212211211175163163163162
Percentage of WeightedAverageNumberOfDilutedSharesOutstanding100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Weighted Average Number Of Shares Outstanding Basic

us-gaap:WeightedAverageNumberOfSharesOutstandingBasic

us-gaap:WeightedAverageNumberOfSharesOutstandingBasic


Weighted
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-12-312013-09-302013-06-302013-03-312012-09-302012-06-302012-03-312011-09-302011-06-302011-03-312010-09-302010-06-302010-03-31
Value151515151515161719202023242525212212212212212211211174162162162162
Percentage of WeightedAverageNumberOfSharesOutstandingBasic100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount

us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount

us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount


Antidilutive
Period End2021-03-312020-03-312014-03-312013-09-302013-06-302013-03-312012-09-302012-06-302012-03-312011-09-302011-06-302011-03-312010-09-302010-06-30
Value1158000.0236580.06900000.07000000.07000000.08600000.05800000.05300000.03000000.03000000.02300000.01100000.03400000.02400000.0
Percentage of AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecurities

us-gaap:Warrant


Antidilutive
Period End2020-09-302020-03-31
Value178330.0114780.0
Percentage of AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount0.048.5

Earnings Per Share Basic

us-gaap:EarningsPerShareBasic

us-gaap:EarningsPerShareBasic


Earnings
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value1.82-0.4-5.17-12.64-3.26-1.67-0.196.793.84.164.636.42.152.873.612.91.512.071.34-1.75-0.61-0.34-0.33-1.390.22-2.050.010.340.090.060.340.290.290.41-0.01
Percentage of EarningsPerShareBasic100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Earnings Per Share Diluted

us-gaap:EarningsPerShareDiluted

us-gaap:EarningsPerShareDiluted


Earnings
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value1.66-0.4-5.17-12.64-3.26-1.67-0.576.343.533.914.446.12.062.743.642.831.482.031.31-29.7-93.91-7.93-5.32-1.13-0.46-0.46-0.59-1.75-0.61-0.34-0.33-1.390.22-2.050.010.330.090.060.340.290.290.41-0.01
Percentage of EarningsPerShareDiluted100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Comprehensive Income Net Of Tax

us-gaap:ComprehensiveIncomeNetOfTax

us-gaap:ComprehensiveIncomeNetOfTax


Comprehensive
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-31
Value32-4.705-105.457-187.082-50.764-42.702-43.173100637510812634658259405157-49.765-178.294-205.842-640.15-2000.57-167.887-109.591-268.401-99.197-98.437-126.769-324.916-125.996-306.98844-438.4738.709751.692-1.99864
Percentage of ComprehensiveIncomeNetOfTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax7.2-1.30.0-48.9-15.9-10.5-7.916.211.113.716.719.95.911.514.89.77.48.510.10.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:ComprehensiveIncomeNetOfTax us-gaap:StatementEquityComponents

us-gaap:AccumulatedOtherComprehensiveIncome


Comprehensive
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312016-12-31
Value4.5841.3374.385-1.44-17.403-6.4770.3983.0942.902-8.4635.89924
Percentage of ComprehensiveIncomeNetOfTax14.1-28.4-2.32.840.8-6.50.64.12.3-24.39.042.2
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax1.00.41.1-0.5-4.3-1.00.10.60.5-1.41.04.2
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:AccumulatedOtherComprehensiveIncome0.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:RetainedEarnings


Comprehensive
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312016-12-31
Value27-6.042-191.467-49.324-25.2991066272123435933
Percentage of ComprehensiveIncomeNetOfTax85.9128.4102.397.259.2106.599.495.997.7124.391.057.8
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.2-1.7-50.1-15.4-6.217.211.013.119.57.310.45.8
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:RetainedEarnings0.00.00.00.00.00.00.00.00.00.00.00.0

Cost Of Goods And Service Excluding Depreciation Depletion And Amortization

us-gaap:CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization

us-gaap:CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization


Cost
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-30
Value355309341345316374492491451438514482474
Percentage of CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax78.986.70.090.499.092.589.679.379.179.079.076.180.1

Disposal Group Not Discontinued Operation Gain Loss On Disposal

us-gaap:DisposalGroupNotDiscontinuedOperationGainLossOnDisposal

us-gaap:DisposalGroupNotDiscontinuedOperationGainLossOnDisposal


Disposal
Period End2020-12-312020-09-302020-06-302019-12-312019-09-302019-06-302018-12-312018-09-302018-06-302017-12-312017-09-302016-12-31
Value0.1360.01.369-9.0080.0-4.3040.00.00.0-0.277210.0
Percentage of DisposalGroupNotDiscontinuedOperationGainLossOnDisposal100.00.0100.0100.00.0100.00.00.00.0100.0100.00.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.00.4-1.60.0-0.80.00.00.0-0.03.50.0

us-gaap:DisposalGroupNotDiscontinuedOperationGainLossOnDisposal us-gaap:DisposalGroupClassification, us-gaap:IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperations

us-gaap:DiscontinuedOperationsDisposedOfBySale, aci:ViperMine


Disposal
Period End2020-12-31
Value0.1
Percentage of DisposalGroupNotDiscontinuedOperationGainLossOnDisposal73.5
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:DiscontinuedOperationsDisposedOfBySale aci:ViperMine0.0

us-gaap:DisposalGroupNotDiscontinuedOperationGainLossOnDisposal us-gaap:IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperations

aci:DalTexAndBriarBranchProperties


Disposal
Period End2020-12-312020-09-302020-06-302019-12-312019-09-302019-06-30
Value0.1360.01.369-9.0080.0-4.304
Percentage of DisposalGroupNotDiscontinuedOperationGainLossOnDisposal100.00.0100.0100.00.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.00.4-1.60.0-0.8
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax aci:DalTexAndBriarBranchProperties0.00.00.00.00.00.0

Gross Profit

us-gaap:GrossProfit

us-gaap:GrossProfit


Gross
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value33-0.337-73.283-187.68-45.286-23.8912310666761018244656373426611-110.521-158.412-44.96447-16.5071432-5.851-6.35-49.842-44.8010.092.505-18.563769546315311817112910711910061
Percentage of GrossProfit100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax7.4-0.10.0-49.1-14.2-5.94.217.211.713.815.613.17.511.311.311.97.811.10.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Income Loss From Continuing Operations Before Income Taxes Extraordinary Items Noncontrolling Interest

us-gaap:IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest

us-gaap:IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest


Income
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-30
Value29-5.664-78.326-191.088-48.118-27.09-8.8111076272827739594666375234-54.691-176.132-207.813-654.416-2343.341-172.174-116.591-101.672-131.568-131.729-167.75-477.558-329.68-129.819-124.68-373.77761-685.666-19.6747131168525471
Percentage of IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.6-1.60.0-50.0-15.1-6.7-1.617.311.013.112.712.36.710.38.310.96.88.76.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Income
Period End2021-03-312020-03-312013-12-312013-09-30
Value-0.5470.2570.5231.75
Percentage of IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest9.7-0.9-0.1-0.5
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-0.20.10.00.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome us-gaap:AccumulatedDefinedBenefitPlansAdjustment0.00.00.00.0

Income Tax Expense Benefit

us-gaap:IncomeTaxExpenseBenefit

us-gaap:IncomeTaxExpenseBenefit


Income
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value2.0060.3780.1990.3791.206-1.791-0.260.3470.0910.07-3.351-45.2-3.366-0.544-34.771-1.6430.3190.841.156-3.27-0.245-1.111-22.048-343.865-4.071-3.396138-34.35-34.869-43.611-104.764-121.913-49.468-54.631-78.35415-250.242-21.079-12.692-5.5830.186124.8257.9415.723-0.775
Percentage of IncomeTaxExpenseBenefit100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.40.10.00.10.4-0.4-0.00.10.00.0-0.5-7.1-0.6-0.1-6.2-0.30.10.10.20.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Interest Expense

us-gaap:InterestExpense

us-gaap:InterestExpense


Interest
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value2.9414.1284.5322.9893.5233.3883.6294.0494.3754.4324.8475.1795.055.3955.5055.9726.0039.42511464544999999999898979695959495887578747577423434373535
Percentage of InterestExpense100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.71.20.00.81.10.80.70.70.80.80.70.80.90.91.01.01.11.62.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:InterestExpense us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedGainLossNetCashFlowHedgeParent


Interest
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value0.6320.6240.6390.6380.6480.2160.13-0.161-0.514-0.515
Percentage of InterestExpense21.515.114.121.318.46.43.6-4.0-11.7-11.6
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.10.20.00.20.20.10.0-0.0-0.1-0.1
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome us-gaap:AccumulatedGainLossNetCashFlowHedgeParent0.00.00.0651.0330.60.00.0-5.4-29.5-142.7

Interest Expense Debt

us-gaap:InterestExpenseDebt us-gaap:DebtInstrument

us-gaap:InterestExpenseDebt

aci:ConvertibleSeniorNotesDue2025


Interest
Period End2021-06-302021-03-312020-12-31
Value3.83.72.3
Percentage of InterestExpenseDebt100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.81.00.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax aci:ConvertibleSeniorNotesDue20250.00.00.0

Interest Income Expense Nonoperating Net

us-gaap:InterestIncomeExpenseNonoperatingNet

us-gaap:InterestIncomeExpenseNonoperatingNet


Interest
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-30
Value-2.794-3.8-4.235-2.53-1.73-2.129-1.878-0.34-2.287-2.289-2.691-3.378-3.498-4.122-4.945-5.252-5.161-8.898-10.754-45.582-44.34-43.313-98.971-99.087-98.612-96.879-96.368-96.268-95.924-94.628-93.959-94.927-93.54-92.251-86.506-74.251-77.64-73.751-74.695-76.85441-33.834-34.082-36.77134
Percentage of InterestIncomeExpenseNonoperatingNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-0.6-1.10.0-0.7-0.5-0.5-0.3-0.1-0.4-0.4-0.4-0.5-0.6-0.7-0.9-0.9-0.9-1.5-1.90.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Net Income Loss

us-gaap:NetIncomeLoss

us-gaap:NetIncomeLoss


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value27-6.042-78.525-191.467-49.324-25.299-8.55110662728612343598168375133-51.421-175.887-206.702-1999.476-168.103-113.195-240.136-97.218-96.86-124.139-371.214-128.363-72.206-70.049-295.42345-435.4891.20659191155474666-1.796
Percentage of NetIncomeLoss100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.2-1.70.0-50.1-15.4-6.2-1.617.211.013.113.219.57.310.414.511.16.88.65.80.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:NetIncomeLoss us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302018-12-312018-09-302018-06-302017-12-312017-09-302017-06-30
Value0.441-0.5470.220.3290.3930.2571.4842.4330.4290.3480.6131.3711.176-0.9050.487
Percentage of NetIncomeLoss1.69.1-0.3-0.2-0.8-1.0-17.42.30.70.40.53.21.4-1.31.3
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.1-0.20.00.10.10.10.30.40.10.10.10.20.2-0.10.1
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome us-gaap:AccumulatedDefinedBenefitPlansAdjustment0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedGainLossNetCashFlowHedgeParent


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value-0.632-0.624-0.54-0.541-0.452-0.2164.2833.1242.2570.876
Percentage of NetIncomeLoss-2.310.30.70.30.90.9-50.12.93.61.2
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-0.1-0.20.0-0.1-0.1-0.10.80.50.40.2
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome us-gaap:AccumulatedGainLossNetCashFlowHedgeParent0.00.00.0-552.0-230.60.00.0105.4129.5242.7

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-09-302013-06-302013-03-31
Value0.0140.01-0.0030.1280.141-0.0010.1350.0350.0150.00.00.0080.0160.00.0550.00.00.3320.6320.0-1.857-0.723-0.881-1.79-0.092-0.719-0.356-0.002-0.16-0.038
Percentage of NetIncomeLoss0.1-0.20.0-0.1-0.30.0-1.60.00.00.00.00.00.00.00.10.00.00.6-1.2-0.00.90.00.51.60.10.70.30.00.20.1
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.00.00.00.0-0.00.00.00.00.00.00.00.00.00.00.00.00.10.00.00.00.00.00.00.00.00.00.00.00.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Nonoperating Income Expense

us-gaap:NonoperatingIncomeExpense

us-gaap:NonoperatingIncomeExpense


Nonoperating
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312013-12-312013-09-302012-12-312012-09-302012-06-302011-12-312011-09-302011-06-302010-12-312010-09-30
Value-0.539-1.527-0.808-0.878-1.102-1.070.0270.975-1.352-1.679-1.056-1.531-1.157-1.6042.268-1.35-0.284-5.579-0.759-20.904-21.271-6.088-16.412-7.482-4.0160.0237-94.927-4.626-74.251-19.0420.0-1.708-49.740.0-6.776
Percentage of NonoperatingIncomeExpense100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.0100.0100.00.0100.0100.00.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-0.1-0.40.0-0.2-0.3-0.30.00.2-0.2-0.3-0.2-0.2-0.2-0.30.4-0.2-0.1-0.9-0.10.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Operating Income Loss

us-gaap:OperatingIncomeLoss

us-gaap:OperatingIncomeLoss


Operating
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value30-4.137-77.518-190.21-47.016-26.02-6.961066474867941615168375846-33.787-154.861-201.725-539.033-2236.772-69.546-19.712-5.303-35.3-35.805-73.122-340.678-234.753-36.279-32.429-282.645135-588.9845413992102102869810632
Percentage of OperatingIncomeLoss100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.8-1.20.0-49.8-14.7-6.4-1.317.111.313.413.312.66.910.69.111.16.99.78.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Profit Loss

us-gaap:ProfitLoss

us-gaap:ProfitLoss


Profit
Period End2021-06-302021-03-312020-06-302020-03-312016-12-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value27-6.042-49.324-25.29933-632.368-1999.476-168.103-113.195-240.135-97.218-96.86-124.139-371.214-128.363-72.206-70.049-295.42345-435.4241.40971191155484666-1.77
Percentage of ProfitLoss100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.2-1.7-15.4-6.25.80.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Revenue From Contract With Customer Excluding Assessed Tax

us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax

us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax


Revenue
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value450357360382319405549619570555650633592575560613549600575
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ConsolidationItems

us-gaap:IntersegmentElimination


Revenue
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value0.1821.2220.9086.177120.9550.6233.2145.1474.2623.6250.0114.733-0.0012.226
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.30.21.93.10.20.10.60.80.70.60.02.00.8-0.00.4
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.30.21.93.10.20.10.60.80.70.60.02.00.8-0.00.4
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:IntersegmentElimination100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.0100.0

us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:OperatingSegments, aci:MetallurgicalSegment


Revenue
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value219177151168138182221254261253302236259238195238227200
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax48.749.742.144.043.545.140.341.145.845.646.537.343.741.434.938.941.434.8
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax48.749.742.144.043.545.140.341.145.845.646.537.343.741.434.938.941.434.8
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments aci:MetallurgicalSegment100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OperatingSegments, aci:PowderRiverBasin


Revenue
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312016-12-31
Value178169180133178222269210212236261229245275
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax50.047.147.341.744.040.643.636.938.336.341.438.842.747.9
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax50.047.147.341.744.040.643.636.938.336.341.438.842.747.9
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments aci:PowderRiverBasin100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OperatingSegments, aci:ThermalSegment


Revenue
Period End2021-06-302020-06-30
Value230174
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax51.254.6
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax51.254.6
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments aci:ThermalSegment100.0100.0

us-gaap:OperatingSegments, us-gaap:AllOtherSegments


Revenue
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value1.22234324131989498851061309991109939197
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.39.48.512.97.818.015.217.215.516.420.616.915.919.515.316.716.9
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.39.48.512.97.818.015.217.215.516.420.616.915.919.515.316.716.9
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments us-gaap:AllOtherSegments100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ConsolidationItems, us-gaap:StatementGeographical

us-gaap:IntersegmentElimination, srt:NorthAmerica


Revenue
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value0.1821.2220.9086.177120.9550.6233.2145.1474.2623.6250.0114.733-0.0012.226
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.30.21.93.10.20.10.60.80.70.60.02.00.8-0.00.4
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.30.21.93.10.20.10.60.80.70.60.02.00.8-0.00.4
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:IntersegmentElimination srt:NorthAmerica100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.0100.0

us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ConsolidationItems, us-gaap:StatementGeographical, us-gaap:StatementBusinessSegments

us-gaap:OperatingSegments, aci:ForeignCountries, aci:MetallurgicalSegment


Revenue
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value172139512098152167190206208259186220208118154145109
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax38.23.726.531.630.937.730.530.836.237.639.929.537.336.321.125.226.419.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax38.23.726.531.630.937.730.530.836.237.639.929.537.336.321.125.226.419.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments aci:ForeignCountries aci:MetallurgicalSegment100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OperatingSegments, aci:ForeignCountries, aci:PowderRiverBasin


Revenue
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312016-12-31
Value1531.3930.7530.00.1810.00.00.00.00.00.00.8431.0680.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax43.10.40.20.00.00.00.00.00.00.00.00.10.20.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax43.10.40.20.00.00.00.00.00.00.00.00.10.20.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments aci:ForeignCountries aci:PowderRiverBasin100.0100.0100.00.0100.00.00.00.00.00.00.0100.0100.00.0

us-gaap:OperatingSegments, aci:ForeignCountries, aci:ThermalSegment


Revenue
Period End2021-06-302020-06-30
Value3010
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.83.2
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.83.2
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments aci:ForeignCountries aci:ThermalSegment100.0100.0

us-gaap:OperatingSegments, aci:ForeignCountries, us-gaap:AllOtherSegments


Revenue
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value9.0024.273103.152504650395174594757172020
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax2.51.13.20.89.37.58.87.17.911.810.08.310.22.83.73.6
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax2.51.13.20.89.37.58.87.17.911.810.08.310.22.83.73.6
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments aci:ForeignCountries us-gaap:AllOtherSegments100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OperatingSegments, srt:NorthAmerica, aci:MetallurgicalSegment


Revenue
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value4716456474029546354444349382977848291
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax10.545.915.712.312.67.39.810.39.68.06.67.86.55.213.813.815.015.8
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax10.545.915.712.312.67.39.810.39.68.06.67.86.55.213.813.815.015.8
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments srt:NorthAmerica aci:MetallurgicalSegment100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OperatingSegments, srt:NorthAmerica, aci:PowderRiverBasin


Revenue
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312016-12-31
Value24168180133178222269210212236261229244275
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.946.747.141.744.040.643.636.938.336.341.438.742.547.9
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.946.747.141.744.040.643.636.938.336.341.438.742.547.9
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments srt:NorthAmerica aci:PowderRiverBasin100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OperatingSegments, srt:NorthAmerica, aci:ThermalSegment


Revenue
Period End2021-06-302020-06-30
Value200164
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax44.551.4
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax44.551.4
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments srt:NorthAmerica aci:ThermalSegment100.0100.0

us-gaap:OperatingSegments, srt:NorthAmerica, us-gaap:AllOtherSegments


Revenue
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value25283128484747465556404352767176
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.97.49.77.18.77.78.48.48.58.96.87.69.312.513.013.3
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.97.49.77.18.77.78.48.48.58.96.87.69.312.513.013.3
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:OperatingSegments srt:NorthAmerica us-gaap:AllOtherSegments100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedGainLossNetCashFlowHedgeParent


Revenue
Period End2020-12-312020-09-302020-06-302019-12-312019-09-302019-06-302019-03-31
Value0.0980.0980.1964.4132.9631.7430.361
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.00.10.80.50.30.1
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.00.10.80.50.30.1
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome us-gaap:AccumulatedGainLossNetCashFlowHedgeParent100.0100.0100.0100.0100.0100.0100.0

us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:StatementGeographical

aci:ForeignCountries


Revenue
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value202167105125108156218237256248311261280257175171165189130
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax45.046.829.332.934.138.639.738.345.044.747.841.347.444.831.328.030.131.622.6
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax45.046.829.332.934.138.639.738.345.044.747.841.347.444.831.328.030.131.622.6
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax aci:ForeignCountries100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

srt:NorthAmerica


Revenue
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value247190254256210248331382313307339371311317384441384445
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax55.053.270.767.165.961.460.361.755.055.352.258.752.655.268.772.069.977.4
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax55.053.270.767.165.961.460.361.755.055.352.258.752.655.268.772.069.977.4
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax srt:NorthAmerica100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Revenues

us-gaap:Revenues

us-gaap:Revenues


Revenues
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value450357360382319405549619570555650633592575560613549600575
Percentage of Revenues100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Selling General And Administrative Expense

us-gaap:SellingGeneralAndAdministrativeExpense

us-gaap:SellingGeneralAndAdministrativeExpense


Selling
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value24211821192221242524262224252221222022201919262524222728292937283433343335302633293028263527
Percentage of SellingGeneralAndAdministrativeExpense100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax5.46.05.15.66.25.64.04.04.44.34.13.64.24.54.03.54.13.54.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Assets

us-gaap:Assets

us-gaap:Assets


Assets
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-10-022016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302009-12-31
Value179417741722165318021857186719341889186318871997194919761979202221322144213621364658468548555106584780368314842986188656882389909531973099201000699359953101651021398551015148994880483448904840
Percentage of Assets100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:Assets us-gaap:ConsolidationItems

aci:CorporateReconcilingItemsAndEliminations


Assetsus-gaap:
Period End2020-12-312019-12-312018-12-312017-12-31
Value714880938906
Percentage of Assets41.547.249.745.8
Percentage of Assets41.547.249.745.8
Percentage of Assets aci:CorporateReconcilingItemsAndEliminations100.0100.0100.0100.0

us-gaap:IntersegmentElimination


Assetsus-gaap:
Period End2021-06-302020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-31
Value6616777057869399109259338849139349971007
Percentage of Assets36.941.039.242.348.648.249.746.745.446.246.246.847.0
Percentage of Assets36.941.039.242.348.648.249.746.745.446.246.246.847.0
Percentage of Assets us-gaap:IntersegmentElimination100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:Assets us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:OperatingSegments, aci:MetallurgicalSegment


Assetsus-gaap:
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-10-02
Value941196811759740693625609605568545561551548548538580575576
Percentage of Assets52.511.147.145.941.137.333.531.532.130.528.928.128.327.827.726.627.226.827.0
Percentage of Assets52.511.147.145.941.137.333.531.532.130.528.928.128.327.827.726.627.226.827.0
Percentage of Assets us-gaap:OperatingSegments aci:MetallurgicalSegment100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OperatingSegments, aci:PowderRiverBasin


Assetsus-gaap:
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-10-02
Value886179189247256256236236230278374379383390446
Percentage of Assets50.010.411.513.813.813.712.212.512.414.718.719.519.419.720.9
Percentage of Assets50.010.411.513.813.813.712.212.512.414.718.719.519.419.720.9
Percentage of Assets us-gaap:OperatingSegments aci:PowderRiverBasin100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OperatingSegments, aci:ThermalSegment


Assetsus-gaap:
Period End2021-06-302020-06-30
Value191356
Percentage of Assets10.719.8
Percentage of Assets10.719.8
Percentage of Assets us-gaap:OperatingSegments aci:ThermalSegment100.0100.0

us-gaap:OperatingSegments, us-gaap:AllOtherSegments


Assetsus-gaap:
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-10-022015-12-312015-09-302014-12-312013-12-31
Value6901626108120105148136139125127134130134130126128129310321339402
Percentage of Assets38.91.01.66.06.55.67.77.27.56.66.46.96.66.86.56.06.06.16.15.54.04.5
Percentage of Assets38.91.01.66.06.55.67.77.27.56.66.46.96.66.86.56.06.06.16.15.54.04.5
Percentage of Assets us-gaap:OperatingSegments us-gaap:AllOtherSegments100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Liabilities

us-gaap:Liabilities

us-gaap:Liabilities


Liabilities
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value14761492143812871254126312271229119111631182131013081288131313501347134313906334631263046351645366426754676166846624669667366949701971377152677068116580662463106613259726322716
Percentage of Liabilities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets82.384.183.577.969.668.065.763.563.162.462.665.667.165.266.466.863.262.665.1136.0134.7129.8124.4110.482.781.280.277.676.575.974.972.972.172.071.568.168.464.764.964.065.153.053.956.1

Liabilities And Stockholders Equity

us-gaap:LiabilitiesAndStockholdersEquity

us-gaap:LiabilitiesAndStockholdersEquity


Liabilities
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value179417741722165318021857186719341889186318871997194919761979202221322144213646584685485551065847803683148429861886568823899095319730992010006993599531016510213985510151489948804840
Percentage of LiabilitiesAndStockholdersEquity100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Stockholders Equity

us-gaap:StockholdersEquity

us-gaap:StockholdersEquity


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-312008-12-312007-12-31
Value318281283365547593640705697699704687640688665672785801746-1676.094-1626.99-1449.128-1244.289-605.441394156016681934203121262253258127102782285431653124357235783533352722912237211517281531
Percentage of StockholdersEquity100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets17.715.916.522.130.432.034.336.536.937.637.434.432.934.833.633.236.837.434.9-36.0-34.7-29.8-24.4-10.417.318.819.822.423.524.125.127.127.928.028.531.931.435.135.035.934.746.845.843.70.00.0

us-gaap:StockholdersEquity us-gaap:StatementEquityComponents

us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-31
Value-26.949-30.912-31.459-4.166-8.33-5.9948.2733943404026231919
Percentage of StockholdersEquity-8.5-11.0-11.1-1.1-1.5-1.01.35.66.25.85.73.83.62.93.0
Percentage of Assets-1.5-1.7-1.8-0.3-0.5-0.30.42.02.32.22.11.31.21.01.0
Percentage of Assets us-gaap:AccumulatedDefinedBenefitPlansAdjustment0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:AccumulatedGainLossNetCashFlowHedgeParent


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312018-12-31
Value-2.564-3.202-3.891-4.394-4.87-5.233-2.5643.328
Percentage of StockholdersEquity-0.8-1.1-1.4-1.2-0.9-0.9-0.40.5
Percentage of Assets-0.1-0.2-0.2-0.3-0.3-0.3-0.10.2
Percentage of Assets us-gaap:AccumulatedGainLossNetCashFlowHedgeParent0.00.00.00.00.00.00.00.0

us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-31
Value-0.267-0.25-0.351-0.2090.046-0.487-0.020.210.3080.034-0.343-0.379-0.481-0.6580.0
Percentage of StockholdersEquity-0.1-0.1-0.1-0.10.0-0.1-0.00.00.00.0-0.0-0.1-0.1-0.10.0
Percentage of Assets-0.0-0.0-0.0-0.00.0-0.0-0.00.00.00.0-0.0-0.0-0.0-0.00.0
Percentage of Assets us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:AccumulatedOtherComprehensiveIncome


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value-29.78-34.364-35.701-8.769-13.154-11.7145.689404646432017262024-1.8153.24137-16.507-7.95-7.95-6.417-19.853-79.096-1.632
Percentage of StockholdersEquity-9.3-12.2-12.6-2.4-2.4-2.00.95.76.76.66.13.02.83.83.13.30.10.21.7-0.60.0-0.2-0.3-0.9-4.6-0.1
Percentage of Assets-1.7-1.9-2.1-0.5-0.7-0.60.32.12.52.52.31.00.91.31.01.1-0.00.00.4-0.20.0-0.1-0.1-0.40.00.0
Percentage of Assets us-gaap:AccumulatedOtherComprehensiveIncome0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:AdditionalPaidInCapital


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value7747707677447397347307347287237177127087037006883054304830383026301530151734172113811358
Percentage of StockholdersEquity243.2273.6270.7203.5134.9123.7114.1104.2104.4103.3101.8103.7110.5102.3105.192.2-245.5182.7134.9106.00.084.377.581.479.988.7
Percentage of Assets43.243.444.645.041.039.539.138.038.638.838.035.736.335.635.432.259.836.233.830.20.029.535.535.60.00.0
Percentage of Assets us-gaap:AdditionalPaidInCapital0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:CommonStock


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value0.2540.2540.2530.2520.2520.2520.2520.250.250.250.250.250.250.250.250.252.1452.1412.1412.1412.1362.1361.6451.6431.4471.436
Percentage of StockholdersEquity0.10.10.10.10.00.00.00.00.00.00.00.00.00.00.00.0-0.20.10.10.10.00.10.10.10.10.1
Percentage of Assets0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0
Percentage of Assets us-gaap:CommonStock0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:RetainedEarnings


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value40037237845764869873174664759252744933329824733-4244.967-1331.825-771.349-104.042622622561465478173
Percentage of StockholdersEquity125.8132.5133.6125.1118.5117.6114.2105.992.884.674.965.452.143.437.14.5341.2-79.8-34.2-3.60.017.425.122.027.711.3
Percentage of Assets22.321.022.027.736.037.639.238.634.331.828.022.517.115.112.51.6-83.1-15.8-8.6-1.00.06.111.59.60.00.0
Percentage of Assets us-gaap:RetainedEarnings0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:TreasuryStock


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value-827.381-827.381-827.381-827.381-827.381-827.381-827.381-816.882-725.524-662.132-583.883-495.232-418.985-340.698-302.1090.0-53.863-53.863-53.848-53.848-53.848-53.848-53.848-53.848-53.8480.0
Percentage of StockholdersEquity-259.8-294.0-291.8-226.3-151.0-139.3-129.2-115.8-104.0-94.6-82.8-72.1-65.4-49.5-45.40.04.3-3.2-2.4-1.90.0-1.5-2.4-2.5-3.10.0
Percentage of Assets-46.1-46.6-48.0-50.0-45.9-44.5-44.3-42.2-38.4-35.5-30.9-24.8-21.5-17.2-15.30.0-1.1-0.6-0.6-0.50.0-0.5-1.1-1.10.00.0
Percentage of Assets us-gaap:TreasuryStock0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Assets Current

us-gaap:AssetsCurrent

us-gaap:AssetsCurrent


Assets
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value55558358856256463970686684083287887785786783086994591187897994410871203131912961530154216251581173717822029175518841914157915681179118211071388798734686
Percentage of AssetsCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets30.932.934.134.131.334.437.844.844.544.746.643.944.043.942.043.044.342.541.121.020.222.423.622.616.118.418.318.918.319.719.821.318.019.019.115.915.811.611.611.213.716.315.114.2

Cash And Cash Equivalents At Carrying Value

us-gaap:CashAndCashEquivalentsAtCarryingValue

us-gaap:CashAndCashEquivalentsAtCarryingValue


Cash
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-312009-12-312008-12-312007-12-31
Value153169187156150105153175232218264245241288273298333347305400269395450494439689734795740865911113364473078455051211713815882699363565061705.08
Percentage of CashAndCashEquivalentsAtCarryingValue100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets8.69.610.99.58.35.78.29.112.311.714.012.312.414.613.814.815.616.214.38.65.88.18.88.55.58.38.79.28.69.810.111.96.67.47.85.55.11.21.41.60.81.41.91.31.20.01.30.00.0

Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents

us-gaap:CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents

us-gaap:CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents


Cash
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-10-01
Value160188193170150105153175232218264245241291273298375416376282
Percentage of CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets8.910.611.210.38.35.78.29.112.311.714.012.312.414.713.814.817.619.417.60.0

Accounts Receivable Net Current

us-gaap:AccountsReceivableNetCurrent

us-gaap:AccountsReceivableNetCurrent


Accounts
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value160129110127118149168206182195200183198208172207190150184165129113117217204198211213203230198190264263247277327295380342335258208190
Percentage of AccountsReceivableNetCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets8.97.36.47.76.68.09.010.79.710.510.69.210.210.58.710.28.97.08.63.62.82.32.33.72.52.42.52.52.42.62.22.02.72.72.52.83.32.93.73.53.35.34.33.9

us-gaap:AccountsReceivableNetCurrent us-gaap:ConcentrationRiskByType

aci:DomesticAndForeignSteelProducers


Accounts
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-31
Value6998126998832767086117
Percentage of AccountsReceivableNetCurrent62.358.663.057.647.727.935.935.635.030.8
Percentage of Assets4.05.36.75.04.10.60.90.80.91.1
Percentage of Assets aci:DomesticAndForeignSteelProducers0.00.00.00.00.00.00.00.00.00.0

Receivables Net Current

us-gaap:ReceivablesNetCurrent us-gaap:ConcentrationRiskByType

us-gaap:ReceivablesNetCurrent

aci:ElectricUtilities


Receivables
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-31
Value416973729683134125159261
Percentage of ReceivablesNetCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets2.43.73.93.74.51.61.61.41.62.6
Percentage of Assets aci:ElectricUtilities0.00.00.00.00.00.00.00.00.00.0

Inventory Net

us-gaap:InventoryNet

us-gaap:InventoryNet


Inventory
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value162154126143154149130171172145125163157143128122137125113159172195196239223240190209228224264287356368365391455488377346350247235240
Percentage of InventoryNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets9.18.77.38.78.68.17.08.99.17.86.68.28.17.36.56.16.45.85.33.43.74.03.94.12.82.92.32.42.62.52.93.03.73.73.73.94.64.83.73.53.45.14.85.0

Other Assets Current

us-gaap:OtherAssetsCurrent

us-gaap:OtherAssetsCurrent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value353958557087971135664759586687063617496363944104515059604954555646608892676796110107121101104113
Percentage of OtherAssetsCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets2.02.23.43.43.94.75.25.93.03.44.04.84.43.53.63.12.93.54.50.80.80.92.10.90.60.70.70.60.60.60.60.50.60.90.90.70.70.91.11.11.22.12.12.3

Property Plant And Equipment Gross

us-gaap:PropertyPlantAndEquipmentGross

us-gaap:PropertyPlantAndEquipmentGross


Property
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-312009-12-31
Value14281342110111031086662698679853106891086857925512
Percentage of PropertyPlantAndEquipmentGross100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets83.071.958.455.750.8129.8117.1109.6106.8106.4118.7113.9

Accumulated Depreciation Depletion And Amortization Property Plant And Equipment

us-gaap:AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment

us-gaap:AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment


Accumulated
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-312009-12-31
Value421357266147323007341431193352291924842145
Percentage of AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets24.519.114.17.41.558.940.534.733.528.650.944.3

Property Plant And Equipment Net

us-gaap:PropertyPlantAndEquipmentNet

us-gaap:PropertyPlantAndEquipmentNet


Property
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value110210581007964106610359848898708488349199259359559801007102710533434349334873619417363416371645365326603661667346778723172727337737273977892794977037726326333083366
Percentage of PropertyPlantAndEquipmentNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets61.459.758.558.359.255.852.746.046.145.644.246.047.547.348.348.547.347.949.373.774.671.870.971.478.976.676.675.876.375.074.971.174.373.373.374.274.377.677.878.276.166.667.869.5

Deferred Tax Assets Liabilities Net

us-gaap:DeferredTaxAssetsLiabilitiesNet

us-gaap:DeferredTaxAssetsLiabilitiesNet


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312013-12-312012-12-312011-12-312010-12-31
Value0.0370.080.17220.113-364.402596-934.702353
Percentage of DeferredTaxAssetsLiabilitiesNet100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets0.00.00.01.10.0-4.16.0-9.27.2

Operating Lease Right Of Use Asset

us-gaap:OperatingLeaseRightOfUseAsset

us-gaap:OperatingLeaseRightOfUseAsset


Operating
Period End2021-06-302021-03-312020-12-312020-09-30
Value15161717
Percentage of OperatingLeaseRightOfUseAsset100.0100.0100.0100.0
Percentage of Assets0.90.91.01.1

Other Assets Noncurrent

us-gaap:OtherAssetsNoncurrent

us-gaap:OtherAssetsNoncurrent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value625755556575697071766895626859657510210858625858119117123130146153158164149154159160180183176173173194117116145
Percentage of OtherAssetsNoncurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets3.53.23.23.43.64.13.73.73.84.13.64.83.23.53.03.33.54.85.11.31.31.21.12.01.51.51.61.71.81.81.81.61.61.61.61.81.81.71.71.81.92.42.43.0

Accounts Payable Current

us-gaap:AccountsPayableCurrent

us-gaap:AccountsPayableCurrent


Accounts
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value1161221039910410813316613813812811913112713413712710395897870128152156167180173160160176207232229224304316294383293286183198128
Percentage of AccountsPayableCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets6.56.96.06.05.85.97.18.67.37.56.86.06.76.46.86.86.04.84.51.91.71.52.52.62.02.02.12.01.91.82.02.22.42.32.23.13.22.93.83.02.83.84.12.7

Accrued Liabilities Current

us-gaap:AccruedLiabilitiesCurrent

us-gaap:AccruedLiabilitiesCurrent


Accrued
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302011-12-31
Value166150155141143143157161162143183203194158184164169172205223153102329310262321302342294328278349307352318355348
Percentage of AccruedLiabilitiesCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets9.38.59.08.58.07.78.48.48.67.79.710.210.08.09.38.28.08.19.64.83.32.16.55.33.33.93.64.03.43.73.13.73.23.53.23.63.4

Debt Current

us-gaap:DebtCurrent

us-gaap:DebtCurrent


Debt
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value13324312525282011131517111214158.5667.4147.898113.3980.00.05107323134362927293323232832115111102280474286970267
Percentage of DebtCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets7.51.41.81.61.41.51.10.60.70.80.90.60.60.70.80.40.30.40.50.10.00.0100.00.60.40.40.40.30.30.30.40.20.20.30.31.21.11.02.70.54.21.41.55.5

Liabilities Current

us-gaap:LiabilitiesCurrent

us-gaap:LiabilitiesCurrent


Liabilities
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value416297290266273280310339314297329334338300334311304283312317231173556449545152351954648151848857956361057777979576310207261115485527631
Percentage of LiabilitiesCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets23.216.816.816.115.215.116.617.616.616.017.516.817.415.216.915.414.313.214.66.84.93.6109.08.55.66.36.26.35.65.95.46.15.86.25.87.88.07.510.07.411.09.910.813.1

Operating Lease Liability Current

us-gaap:OperatingLeaseLiabilityCurrent

us-gaap:OperatingLeaseLiabilityCurrent


Operating
Period End2021-06-302021-03-312020-12-312020-09-30
Value2486000.02476000.02454000.02427000.0
Percentage of OperatingLeaseLiabilityCurrent100.0100.0100.0100.0
Percentage of Assets0.10.10.10.1

Other Accrued Liabilities Current

us-gaap:OtherAccruedLiabilitiesCurrent

us-gaap:OtherAccruedLiabilitiesCurrent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302011-12-312010-12-31
Value2115149.8121110109.9519.8766.038175.63348.736149.0465.3016.4126.1948.6178.9718.90511282837352728242620161718101712
Percentage of OtherAccruedLiabilitiesCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets1.20.90.80.60.60.60.60.50.50.30.90.31.80.40.70.40.20.30.30.20.20.20.20.50.40.40.40.30.30.30.30.20.20.20.20.10.20.2

Long Term Debt

us-gaap:LongTermDebt

us-gaap:LongTermDebt


Long
Period End2021-06-302021-03-312020-12-312020-09-302019-12-31
Value535543508394310
Percentage of LongTermDebt100.0100.0100.0100.0100.0
Percentage of Assets29.930.729.523.816.6

Long Term Debt Noncurrent

us-gaap:LongTermDebtNoncurrent

us-gaap:LongTermDebtNoncurrent


Long
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value402519477368323330290292295297300302305307310312315318351300.00.030510851145117512351265116511251185074507850825085446544643967376238413773153915381540
Percentage of LongTermDebtNoncurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets22.429.327.722.318.017.815.515.115.616.015.915.215.715.615.715.514.814.816.50.60.00.00.687.463.661.660.859.559.157.956.953.252.251.250.844.944.939.036.839.037.231.431.531.8

Pension And Other Postretirement Defined Benefit Plans Liabilities Noncurrent

us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent us-gaap:RetirementPlanType

us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Pension
Period End2020-12-312019-12-312018-12-312017-12-31
Value948083102
Percentage of PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent100.0100.0100.0100.0
Percentage of Assets5.54.34.45.2
Percentage of Assets us-gaap:OtherPostretirementBenefitPlansDefinedBenefit0.00.00.00.0

us-gaap:PensionPlansDefinedBenefit


Pension
Period End2020-12-312019-12-312018-12-312017-12-31
Value2.8795.4761614
Percentage of PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent3.16.819.413.7
Percentage of Assets0.20.30.90.7
Percentage of Assets us-gaap:PensionPlansDefinedBenefit0.00.00.00.0

Other Liabilities Noncurrent

us-gaap:OtherLiabilitiesNoncurrent

us-gaap:OtherLiabilitiesNoncurrent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value971039810298958293806848614964597665776365331811924810914115318117218119020520522722118222326825527725812411098
Percentage of OtherLiabilitiesNoncurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets5.45.95.76.25.55.24.44.84.23.72.63.12.53.23.03.83.13.63.01.40.70.42.34.31.41.71.82.12.02.12.12.22.12.32.21.82.22.62.52.82.52.52.32.0

Operating Lease Liability Noncurrent

us-gaap:OperatingLeaseLiabilityNoncurrent

us-gaap:OperatingLeaseLiabilityNoncurrent


Operating
Period End2021-06-302021-03-312020-12-312020-09-30
Value13141515
Percentage of OperatingLeaseLiabilityNoncurrent100.0100.0100.0100.0
Percentage of Assets0.80.80.91.0

Lessee Operating Lease Liability Payments Due Year Two

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearTwo

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearTwo


Lessee
Period End2021-06-302021-03-312020-12-312020-09-30
Value3285000.03285000.03317000.03317000.0
Percentage of LesseeOperatingLeaseLiabilityPaymentsDueYearTwo100.0100.0100.0100.0
Percentage of Assets0.20.20.20.2

Lessee Operating Lease Liability Payments Due Year Three

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearThree

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearThree


Lessee
Period End2021-06-302021-03-312020-12-312020-09-30
Value3200000.03200000.03285000.03285000.0
Percentage of LesseeOperatingLeaseLiabilityPaymentsDueYearThree100.0100.0100.0100.0
Percentage of Assets0.20.20.20.2

Lessee Operating Lease Liability Payments Due Year Four

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearFour

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearFour


Lessee
Period End2021-06-302021-03-312020-12-312020-09-30
Value3185000.03185000.03200000.03200000.0
Percentage of LesseeOperatingLeaseLiabilityPaymentsDueYearFour100.0100.0100.0100.0
Percentage of Assets0.20.20.20.2

Lessee Operating Lease Liability Payments Due Year Five

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearFive

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueYearFive


Lessee
Period End2020-12-312020-09-30
Value3185000.07798000.0
Percentage of LesseeOperatingLeaseLiabilityPaymentsDueYearFive100.0100.0
Percentage of Assets0.20.5

Lessee Operating Lease Liability Payments Due After Year Five

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive


Lessee
Period End2020-12-31
Value4614000.0
Percentage of LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFive100.0
Percentage of Assets0.3

Operating Lease Liability

us-gaap:OperatingLeaseLiability

us-gaap:OperatingLeaseLiability


Operating
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-30
Value161717181819192021
Percentage of OperatingLeaseLiability100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets0.91.01.01.11.01.11.11.11.2

Lessee Operating Lease Liability Payments Due

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDue

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDue


Lessee
Period End2021-06-302021-03-312020-12-312020-09-30
Value19202021
Percentage of LesseeOperatingLeaseLiabilityPaymentsDue100.0100.0100.0100.0
Percentage of Assets1.11.11.21.3

Lessee Operating Lease Liability Undiscounted Excess Amount

us-gaap:LesseeOperatingLeaseLiabilityUndiscountedExcessAmount

us-gaap:LesseeOperatingLeaseLiabilityUndiscountedExcessAmount


Lessee
Period End2021-06-302021-03-312020-12-312020-09-30
Value2853000.02995000.03236000.03485000.0
Percentage of LesseeOperatingLeaseLiabilityUndiscountedExcessAmount100.0100.0100.0100.0
Percentage of Assets0.20.20.20.2

Stockholders Equity

us-gaap:StockholdersEquity

us-gaap:StockholdersEquity


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-312008-12-312007-12-31
Value318281283365547593640705697699704687640688665672785801746-1676.094-1626.99-1449.128-1244.289-605.441394156016681934203121262253258127102782285431653124357235783533352722912237211517281531
Percentage of StockholdersEquity100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets17.715.916.522.130.432.034.336.536.937.637.434.432.934.833.633.236.837.434.9-36.0-34.7-29.8-24.4-10.417.318.819.822.423.524.125.127.127.928.028.531.931.435.135.035.934.746.845.843.70.00.0

us-gaap:StockholdersEquity us-gaap:StatementEquityComponents

us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-31
Value-26.949-30.912-31.459-4.166-8.33-5.9948.2733943404026231919
Percentage of StockholdersEquity-8.5-11.0-11.1-1.1-1.5-1.01.35.66.25.85.73.83.62.93.0
Percentage of Assets-1.5-1.7-1.8-0.3-0.5-0.30.42.02.32.22.11.31.21.01.0
Percentage of Assets us-gaap:AccumulatedDefinedBenefitPlansAdjustment0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:AccumulatedGainLossNetCashFlowHedgeParent


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312018-12-31
Value-2.564-3.202-3.891-4.394-4.87-5.233-2.5643.328
Percentage of StockholdersEquity-0.8-1.1-1.4-1.2-0.9-0.9-0.40.5
Percentage of Assets-0.1-0.2-0.2-0.3-0.3-0.3-0.10.2
Percentage of Assets us-gaap:AccumulatedGainLossNetCashFlowHedgeParent0.00.00.00.00.00.00.00.0

us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-31
Value-0.267-0.25-0.351-0.2090.046-0.487-0.020.210.3080.034-0.343-0.379-0.481-0.6580.0
Percentage of StockholdersEquity-0.1-0.1-0.1-0.10.0-0.1-0.00.00.00.0-0.0-0.1-0.1-0.10.0
Percentage of Assets-0.0-0.0-0.0-0.00.0-0.0-0.00.00.00.0-0.0-0.0-0.0-0.00.0
Percentage of Assets us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:AccumulatedOtherComprehensiveIncome


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value-29.78-34.364-35.701-8.769-13.154-11.7145.689404646432017262024-1.8153.24137-16.507-7.95-7.95-6.417-19.853-79.096-1.632
Percentage of StockholdersEquity-9.3-12.2-12.6-2.4-2.4-2.00.95.76.76.66.13.02.83.83.13.30.10.21.7-0.60.0-0.2-0.3-0.9-4.6-0.1
Percentage of Assets-1.7-1.9-2.1-0.5-0.7-0.60.32.12.52.52.31.00.91.31.01.1-0.00.00.4-0.20.0-0.1-0.1-0.40.00.0
Percentage of Assets us-gaap:AccumulatedOtherComprehensiveIncome0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:AdditionalPaidInCapital


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value7747707677447397347307347287237177127087037006883054304830383026301530151734172113811358
Percentage of StockholdersEquity243.2273.6270.7203.5134.9123.7114.1104.2104.4103.3101.8103.7110.5102.3105.192.2-245.5182.7134.9106.00.084.377.581.479.988.7
Percentage of Assets43.243.444.645.041.039.539.138.038.638.838.035.736.335.635.432.259.836.233.830.20.029.535.535.60.00.0
Percentage of Assets us-gaap:AdditionalPaidInCapital0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:CommonStock


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value0.2540.2540.2530.2520.2520.2520.2520.250.250.250.250.250.250.250.250.252.1452.1412.1412.1412.1362.1361.6451.6431.4471.436
Percentage of StockholdersEquity0.10.10.10.10.00.00.00.00.00.00.00.00.00.00.00.0-0.20.10.10.10.00.10.10.10.10.1
Percentage of Assets0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0
Percentage of Assets us-gaap:CommonStock0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:RetainedEarnings


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value40037237845764869873174664759252744933329824733-4244.967-1331.825-771.349-104.042622622561465478173
Percentage of StockholdersEquity125.8132.5133.6125.1118.5117.6114.2105.992.884.674.965.452.143.437.14.5341.2-79.8-34.2-3.60.017.425.122.027.711.3
Percentage of Assets22.321.022.027.736.037.639.238.634.331.828.022.517.115.112.51.6-83.1-15.8-8.6-1.00.06.111.59.60.00.0
Percentage of Assets us-gaap:RetainedEarnings0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:TreasuryStock


Stockholders
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312012-01-012011-12-312010-12-312009-12-312008-12-312007-12-31
Value-827.381-827.381-827.381-827.381-827.381-827.381-827.381-816.882-725.524-662.132-583.883-495.232-418.985-340.698-302.1090.0-53.863-53.863-53.848-53.848-53.848-53.848-53.848-53.848-53.8480.0
Percentage of StockholdersEquity-259.8-294.0-291.8-226.3-151.0-139.3-129.2-115.8-104.0-94.6-82.8-72.1-65.4-49.5-45.40.04.3-3.2-2.4-1.90.0-1.5-2.4-2.5-3.10.0
Percentage of Assets-46.1-46.6-48.0-50.0-45.9-44.5-44.3-42.2-38.4-35.5-30.9-24.8-21.5-17.2-15.30.0-1.1-0.6-0.6-0.50.0-0.5-1.1-1.10.00.0
Percentage of Assets us-gaap:TreasuryStock0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Common Stock Value

us-gaap:CommonStockValue

us-gaap:CommonStockValue


Common
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value254000.0254000.0253000.0252000.0252000.0252000.0252000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.0250000.02145000.02145000.02145000.02145000.02145000.02145000.02145000.02141000.02141000.02141000.02141000.02141000.02141000.02141000.02141000.02141000.02141000.02138000.02141000.02136000.02135000.02127000.01647000.01645000.01643000.0
Percentage of CommonStockValue100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Additional Paid In Capital Common Stock

us-gaap:AdditionalPaidInCapitalCommonStock

us-gaap:AdditionalPaidInCapitalCommonStock


Additional
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value7747707677447397347307347287237177127087037006976946926883056305530553054305230513050304830463044304030383035303230293026302430223024301530122991174017341721
Percentage of AdditionalPaidInCapitalCommonStock100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets43.243.444.645.041.039.539.138.038.638.838.035.736.335.635.434.532.632.332.265.665.262.959.852.238.036.736.235.335.234.533.831.831.230.530.230.430.429.829.530.629.535.535.535.6

Retained Earnings Accumulated Deficit

us-gaap:RetainedEarningsAccumulatedDeficit

us-gaap:RetainedEarningsAccumulatedDeficit


Retained
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value4003723784576486987317466475925274493332982471731138533-4678.977-4627.556-4451.669-4244.967-3612.599-1613.123-1445.02-1331.825-1091.689-994.471-897.611-771.349-393.765-259.032-180.459-104.042197158600622586593600561465
Percentage of RetainedEarningsAccumulatedDeficit100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets22.321.022.027.736.037.639.238.634.331.828.022.517.115.112.58.65.34.01.6-100.4-98.8-91.7-83.1-61.8-20.1-17.4-15.8-12.7-11.5-10.2-8.6-4.1-2.7-1.8-1.02.01.65.96.15.95.912.311.59.6

Accumulated Other Comprehensive Income Loss Net Of Tax

us-gaap:AccumulatedOtherComprehensiveIncomeLossNetOfTax

us-gaap:AccumulatedOtherComprehensiveIncomeLossNetOfTax


Accumulated
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value-29.78-34.364-35.701-8.769-13.154-11.7145.689404646432017262018272424-1.706-3.362-0.955-1.8155.9677.0616.8453.24131333537-8.606-10.973-15.016-16.507-5.21-3.758-0.651-7.95-13.775-6.3482.28-6.417-19.853
Percentage of AccumulatedOtherComprehensiveIncomeLossNetOfTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets-1.7-1.9-2.1-0.5-0.7-0.60.32.12.52.52.31.00.91.31.00.91.31.11.1-0.0-0.1-0.0-0.00.10.10.10.00.40.40.40.4-0.1-0.1-0.2-0.2-0.1-0.0-0.0-0.1-0.1-0.10.0-0.1-0.4

Treasury Stock Value

us-gaap:TreasuryStockValue

us-gaap:TreasuryStockValue


Treasury
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value827827827827827827827816725662583495418340302217510.053535353535353535353535353535353535353535353535353
Percentage of TreasuryStockValue100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets46.146.648.050.045.944.544.342.238.435.530.924.821.517.215.310.82.40.01.21.11.11.10.90.70.60.60.60.60.60.60.60.60.50.50.50.50.50.50.50.51.11.11.1

Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value

us-gaap:AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue

us-gaap:AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue


Adjustments
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312016-12-31
Value4.6363.9034.9714.8913.9625.8335.8225.6514.1684.1473.8451.032
Percentage of AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets0.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue us-gaap:StatementEquityComponents

us-gaap:AdditionalPaidInCapital


Adjustments
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312016-12-31
Value4.6133.8854.9714.8913.9625.8335.8225.6514.1684.1473.8451.032
Percentage of AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue99.599.5100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of Assets0.00.00.00.00.00.00.00.00.00.00.00.0
Percentage of Assets us-gaap:AdditionalPaidInCapital0.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:RetainedEarnings


Adjustments
Period End2021-06-302021-03-31
Value0.0230.018
Percentage of AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue0.50.5
Percentage of Assets0.00.0
Percentage of Assets us-gaap:RetainedEarnings0.00.0

Net Cash Provided By Used In Operating Activities

us-gaap:NetCashProvidedByUsedInOperatingActivities

us-gaap:NetCashProvidedByUsedInOperatingActivities


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value205.6865.1922937-12.0358510814084151121786694101751258429-50.154-10.0983545-121.302-4.273-35.57280-38.071-40.275-130.851348.75443-23.59126140541701572288624019716693
Percentage of NetCashProvidedByUsedInOperatingActivities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax4.51.61.47.811.9-3.015.617.524.715.323.319.113.211.616.816.513.720.914.60.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Net Cash Provided By Used In Investing Activities

us-gaap:NetCashProvidedByUsedInInvestingActivities

us-gaap:NetCashProvidedByUsedInInvestingActivities


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value-38.006-42.765-114.064-53.65316-74.88-85.109-64.309-47.493-42.2-39.686-28.94-36.8071.48140-2.441-52.075-46.13517104-70.144-19.259-103.62730-80.755-26.769-17.776-19.704-76.6898.413-81.033366-78.436-81.338-235.689-220.611-109.116-83.75-322.91-19.227-3061.25-93.529-127.343-52.342-144.153-65.291
Percentage of NetCashProvidedByUsedInInvestingActivities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-8.4-12.0-31.6-14.05.2-18.5-15.5-10.4-8.3-7.6-6.1-4.6-6.20.37.3-0.4-9.5-7.73.10.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Net Cash Provided By Used In Financing Activities

us-gaap:NetCashProvidedByUsedInFinancingActivities

us-gaap:NetCashProvidedByUsedInFinancingActivities


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value-10.3523213144-9.6839-22.96-100.839-79.75-88.971-92.904-88.09-90.99-50.692-89.079-175.869-64.456-39.2522.709-3.165-5.286-26.02624-21.57-48.26-13.217-8.355-4.852-10.847-13.476-10.146-11.876-16.232-16.456493-2.2474638.381131-61.7762846-16.989-83.284-138.196-15.279-38.804
Percentage of NetCashProvidedByUsedInFinancingActivities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-2.39.036.511.6-3.09.6-4.2-16.3-14.0-16.0-14.3-13.9-15.4-8.8-15.9-28.7-11.7-6.50.50.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Period Increase Decrease Including Exchange Rate Effect

us-gaap:CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect

us-gaap:CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect


Cash
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value-27.935-4.89222044-47.863-22.408-57.00113-46.187194.089-49.66817-24.737-76.964-41.2614094
Percentage of CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-6.2-1.46.35.414.0-11.8-4.1-9.22.4-8.33.00.6-8.43.1-4.4-12.5-7.56.716.4

Depreciation Depletion And Amortization

us-gaap:DepreciationDepletionAndAmortization

us-gaap:DepreciationDepletionAndAmortization


Depreciation
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value27252732303129302625273130292731303132695863731039710410610510210498111118118125126132139164123958395928788
Percentage of DepreciationDepletionAndAmortization100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DepreciationDepletionAndAmortization us-gaap:ConsolidationItems

us-gaap:IntersegmentElimination


Depreciation
Period End2021-06-302020-09-302020-06-302020-03-312019-09-302019-06-302019-03-31
Value0.2370.2590.2621.2460.4790.6230.591
Percentage of DepreciationDepletionAndAmortization0.80.80.94.01.62.32.3

us-gaap:DepreciationDepletionAndAmortization us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:OperatingSegments, aci:MetallurgicalSegment


Depreciation
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value224.6882224222220191716
Percentage of DepreciationDepletionAndAmortization81.218.280.874.273.971.969.866.065.464.8

us-gaap:OperatingSegments, aci:PowderRiverBasin


Depreciation
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value205.0385.8475.2835.2085.1085.9564.884.865
Percentage of DepreciationDepletionAndAmortization80.918.417.917.516.617.419.718.419.2

us-gaap:OperatingSegments, aci:ThermalSegment


Depreciation
Period End2021-06-302020-06-30
Value4.9937.616
Percentage of DepreciationDepletionAndAmortization17.925.2

us-gaap:OperatingSegments, us-gaap:AllOtherSegments


Depreciation
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value0.2270.0032.3032.3332.3373.4383.8523.6893.435
Percentage of DepreciationDepletionAndAmortization0.90.07.17.77.511.712.713.913.6

Increase Decrease In Inventories

us-gaap:IncreaseDecreaseInInventories

us-gaap:IncreaseDecreaseInInventories


Increase
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value8.13628-3.373-11.2934.70419-30.822-1.2972720-38.4086.67313146.524-10.4921111-24.87-13.452-20.418-0.734-42.31415-16.11749-18.959-19.0673.919-7.441-23.248-12.616-11.5872.816-26.103-63.564-30.99711127-13.397241216-19.36-4.842.382
Percentage of IncreaseDecreaseInInventories100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Increase Decrease In Other Operating Capital Net

us-gaap:IncreaseDecreaseInOtherOperatingCapitalNet

us-gaap:IncreaseDecreaseInOtherOperatingCapitalNet


Increase
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value-17.285-8.621-3.104-19.269-2.095-16.865-0.214-13.5962.861-6.19744-1.22917-8.005180.945-14.593-6.96422-22.26821-15.508236.72322-25.64642-29.5122.773-10.52226-13.446-9.984-16.307-72.0946918-4.10214-10.9498.723-23.72936-25.1764.896-23.627
Percentage of IncreaseDecreaseInOtherOperatingCapitalNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Net Cash Provided By Used In Operating Activities

us-gaap:NetCashProvidedByUsedInOperatingActivities

us-gaap:NetCashProvidedByUsedInOperatingActivities


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value205.6865.1922937-12.0358510814084151121786694101751258429-50.154-10.0983545-121.302-4.273-35.57280-38.071-40.275-130.851348.75443-23.59126140541701572288624019716693
Percentage of NetCashProvidedByUsedInOperatingActivities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax4.51.61.47.811.9-3.015.617.524.715.323.319.113.211.616.816.513.720.914.60.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Net Income Loss

us-gaap:NetIncomeLoss

us-gaap:NetIncomeLoss


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value27-6.042-78.525-191.467-49.324-25.299-8.55110662728612343598168375133-51.421-175.887-206.702-1999.476-168.103-113.195-240.136-97.218-96.86-124.139-371.214-128.363-72.206-70.049-295.42345-435.4891.20659191155474666-1.796
Percentage of NetIncomeLoss100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.2-1.7-21.8-50.1-15.4-6.2-1.617.211.013.113.219.57.310.414.511.16.88.65.80.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:NetIncomeLoss us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302018-12-312018-09-302018-06-302017-12-312017-09-302017-06-30
Value0.441-0.5470.220.3290.3930.2571.4842.4330.4290.3480.6131.3711.176-0.9050.487
Percentage of NetIncomeLoss1.69.1-0.3-0.2-0.8-1.0-17.42.30.70.40.53.21.4-1.31.3
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.1-0.20.10.10.10.10.30.40.10.10.10.20.2-0.10.1
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome us-gaap:AccumulatedDefinedBenefitPlansAdjustment0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedGainLossNetCashFlowHedgeParent


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value-0.632-0.624-0.54-0.541-0.452-0.2164.2833.1242.2570.876
Percentage of NetIncomeLoss-2.310.30.70.30.90.9-50.12.93.61.2
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-0.1-0.2-0.1-0.1-0.1-0.10.80.50.40.2
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome us-gaap:AccumulatedGainLossNetCashFlowHedgeParent0.00.0-551.0-552.0-230.60.097.1105.4129.5242.7

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-09-302013-06-302013-03-31
Value0.0140.01-0.0030.1280.141-0.0010.1350.0350.0150.00.00.0080.0160.00.0550.00.00.3320.6320.0-1.857-0.723-0.881-1.79-0.092-0.719-0.356-0.002-0.16-0.038
Percentage of NetIncomeLoss0.1-0.20.0-0.1-0.30.0-1.60.00.00.00.00.00.00.00.10.00.00.6-1.2-0.00.90.00.51.60.10.70.30.00.20.1
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax0.00.0-0.00.00.0-0.00.00.00.00.00.00.00.00.00.00.00.00.10.00.00.00.00.00.00.00.00.00.00.00.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss0.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Profit Loss

us-gaap:ProfitLoss

us-gaap:ProfitLoss


Profit
Period End2021-06-302021-03-312020-06-302020-03-312016-12-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value27-6.042-49.324-25.29933-632.368-1999.476-168.103-113.195-240.135-97.218-96.86-124.139-371.214-128.363-72.206-70.049-295.42345-435.4241.40971191155484666-1.77
Percentage of ProfitLoss100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax6.2-1.7-15.4-6.25.80.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Share Based Compensation

us-gaap:ShareBasedCompensation

us-gaap:ShareBasedCompensation


Share
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value4.6133.8853.5284.9714.9743.9624.6845.8325.8225.6515.3584.1694.1473.8452.9522.5432.5162.4261.0320.6610.4321.0031.3011.1051.5941.762.1582.223.1362.3332.8813.1053.0912.7132.3872.4212.9354.0791.8631.9481.7815.292.0772.2013.7553.684
Percentage of ShareBasedCompensation100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Net Cash Provided By Used In Investing Activities

us-gaap:NetCashProvidedByUsedInInvestingActivities

us-gaap:NetCashProvidedByUsedInInvestingActivities


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value-38.006-42.765-114.064-53.65316-74.88-85.109-64.309-47.493-42.2-39.686-28.94-36.8071.48140-2.441-52.075-46.13517104-70.144-19.259-103.62730-80.755-26.769-17.776-19.704-76.6898.413-81.033366-78.436-81.338-235.689-220.611-109.116-83.75-322.91-19.227-3061.25-93.529-127.343-52.342-144.153-65.291
Percentage of NetCashProvidedByUsedInInvestingActivities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-8.4-12.0-31.6-14.05.2-18.5-15.5-10.4-8.3-7.6-6.1-4.6-6.20.37.3-0.4-9.5-7.73.10.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Payments To Acquire Property Plant And Equipment

us-gaap:PaymentsToAcquirePropertyPlantAndEquipment

us-gaap:PaymentsToAcquirePropertyPlantAndEquipment


Payments
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value7176805760871284948393925209.4532813105.95158.296685.9269.7749.88976222822811473541145491101108933251086938934913931
Percentage of PaymentsToAcquirePropertyPlantAndEquipment100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:PaymentsToAcquirePropertyPlantAndEquipment us-gaap:ConsolidationItems

us-gaap:IntersegmentElimination


Payments
Period End2021-06-302020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302015-09-30
Value3.801-0.8921.2582.3290.7381.1381.2591.0763.0731.722.9241.4260.4710.142.141
Percentage of PaymentsToAcquirePropertyPlantAndEquipment5.3-1.62.12.71.52.33.24.214.918.221.513.07.91.721.7

us-gaap:PaymentsToAcquirePropertyPlantAndEquipment us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:OperatingSegments, aci:MetallurgicalSegment


Payments
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value660.288755757781123631312817115.829164.5976.8254.6113
Percentage of PaymentsToAcquirePropertyPlantAndEquipment93.90.494.4100.694.589.787.573.864.079.872.769.457.861.758.033.862.277.587.6

us-gaap:OperatingSegments, aci:PowderRiverBasin


Payments
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312016-12-31
Value760.420.0261.1453.097105.402130.4144.9193.4583.0650.6980.934
Percentage of PaymentsToAcquirePropertyPlantAndEquipment99.00.50.01.93.58.110.927.11.112.413.514.97.46.1

us-gaap:OperatingSegments, aci:ThermalSegment


Payments
Period End2021-06-302020-06-30
Value0.5752.1
Percentage of PaymentsToAcquirePropertyPlantAndEquipment0.83.4

us-gaap:OperatingSegments, us-gaap:AllOtherSegments


Payments
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-30
Value0.4490.9180.5420.9553.6163.7896.8373.2116.254.9023.3322.5591.2065.2484.0131.8990.7410.6841.00.9491.9613.9362.8891.6682.6424.3674.6782.358
Percentage of PaymentsToAcquirePropertyPlantAndEquipment0.61.10.91.64.12.913.86.616.012.413.012.412.818.329.517.312.54.512.11.433.140.329.22.211.515.320.42.9

Net Cash Provided By Used In Financing Activities

us-gaap:NetCashProvidedByUsedInFinancingActivities

us-gaap:NetCashProvidedByUsedInFinancingActivities


Net
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value-10.3523213144-9.6839-22.96-100.839-79.75-88.971-92.904-88.09-90.99-50.692-89.079-175.869-64.456-39.2522.709-3.165-5.286-26.02624-21.57-48.26-13.217-8.355-4.852-10.847-13.476-10.146-11.876-16.232-16.456493-2.2474638.381131-61.7762846-16.989-83.284-138.196-15.279-38.804
Percentage of NetCashProvidedByUsedInFinancingActivities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0
Percentage of RevenueFromContractWithCustomerExcludingAssessedTax-2.39.036.511.6-3.09.6-4.2-16.3-14.0-16.0-14.3-13.9-15.4-8.8-15.9-28.7-11.7-6.50.50.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.0

Debt Instrument Interest Rate Stated Percentage

us-gaap:DebtInstrumentInterestRateStatedPercentage us-gaap:DebtInstrument

us-gaap:DebtInstrumentInterestRateStatedPercentage

aci:ConvertibleSeniorNotesDue2025


Debt
Period End2020-11-03
Value5.25
Percentage of DebtInstrumentInterestRateStatedPercentage100.0
Percentage of LongTermDebtAndCapitalLeaseObligations0.0
Percentage of LongTermDebtAndCapitalLeaseObligations aci:ConvertibleSeniorNotesDue20250.0

aci:TaxExemptBonds


Debt
Period End2020-07-02
Value5.0
Percentage of DebtInstrumentInterestRateStatedPercentage0.0
Percentage of LongTermDebtAndCapitalLeaseObligations0.0
Percentage of LongTermDebtAndCapitalLeaseObligations aci:TaxExemptBonds0.0

us-gaap:DebtInstrumentInterestRateStatedPercentage us-gaap:DebtInstrument, us-gaap:SubsequentEventType

us-gaap:Bonds, us-gaap:SubsequentEvent


Debt
Period End2020-07-02
Value0.05
Percentage of DebtInstrumentInterestRateStatedPercentage0.0
Percentage of LongTermDebtAndCapitalLeaseObligations0.0
Percentage of LongTermDebtAndCapitalLeaseObligations us-gaap:Bonds us-gaap:SubsequentEvent0.0

Deferred Tax Assets Net

us-gaap:DeferredTaxAssetsNet

us-gaap:DeferredTaxAssetsNet


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-31
Value144.3214.361272143510921113976783512
Percentage of DeferredTaxAssetsNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Unrecognized Tax Benefits

us-gaap:UnrecognizedTaxBenefits

us-gaap:UnrecognizedTaxBenefits


Unrecognized
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-31
Value483939117.45538
Percentage of UnrecognizedTaxBenefits100.0100.0100.0100.0100.0100.0

us-gaap:UnrecognizedTaxBenefits us-gaap:StatementScenario

srt:ScenarioForecast


Unrecognized
Period End2021-12-312020-12-31
Value0.10.5
Percentage of UnrecognizedTaxBenefits0.01.0

Deferred Tax Assets Valuation Allowance

us-gaap:DeferredTaxAssetsValuationAllowance

us-gaap:DeferredTaxAssetsValuationAllowance


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-31
Value5735065306101021113527043342.8310.737
Percentage of DeferredTaxAssetsValuationAllowance100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:AdjustedEBITDA

aci:AdjustedEBITDA


Adjusted
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-31
Value66304.13417-10.7321243106105107122124851041059512181-8.36-10.0261344581716427
Percentage of AdjustedEBITDA100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:AdjustedEBITDA us-gaap:ConsolidationItems

us-gaap:IntersegmentElimination


Adjusted
Period End2021-06-302020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302015-09-30
Value-36.492-26.597-21.528-27.903-31.005-21.99-10.982-30.202-39.605-22.0-18.24-25.651-22.339-24.51-6.08
Percentage of AdjustedEBITDA-54.9-152.6200.6-216.1-29.1-20.8-10.2-24.2-46.4-21.0-17.4-26.8-18.4-30.1-4.5

aci:AdjustedEBITDA us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:OperatingSegments, aci:MetallurgicalSegment


Adjusted
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-31
Value61131512204241701019197818683536268
Percentage of AdjustedEBITDA92.142.3369.771.2-194.8330.893.966.496.685.379.865.1101.579.850.865.456.4

us-gaap:OperatingSegments, aci:PowderRiverBasin


Adjusted
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-31
Value412134-5.362-0.5822550142023482627
Percentage of AdjustedEBITDA134.6525.0197.950.0-4.557.447.013.919.219.538.931.026.2

us-gaap:OperatingSegments, aci:ThermalSegment


Adjusted
Period End2021-06-302020-06-30
Value41-10.114
Percentage of AdjustedEBITDA62.894.2

us-gaap:OperatingSegments, us-gaap:AllOtherSegments


Adjusted
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-30
Value-23.781-7.269-2.87-4.752-1.327.79616106.11915251115212627200.595-3.328127.4561.6912217
Percentage of AdjustedEBITDA-77.0-175.8-16.544.3-10.217.815.610.35.713.020.213.914.920.228.122.525.8-7.133.29.616.42.131.226.9

aci:AnnualPurchaseOfCommodityForOperations us-gaap:Range, us-gaap:ConcentrationRiskByType, us-gaap:HedgingDesignation

aci:AnnualPurchaseOfCommodityForOperations

srt:Maximum, aci:DieselPurchases, us-gaap:Nondesignated


Annual
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-30
Value0.0350.00.0350.00.0471.0
Percentage of AnnualPurchaseOfCommodityForOperations0.0100.00.00.0100.00.00.0100.0100.0

srt:Minimum, aci:DieselPurchases, us-gaap:Nondesignated


Annual
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-30
Value0.0300.00.0300.00.0400.0
Percentage of AnnualPurchaseOfCommodityForOperations0.085.70.00.085.70.00.085.10.0

aci:AssetImpairmentChargesAndMineClosureCosts us-gaap:ScheduleOfEquityMethodInvestmentEquityMethodInvesteeName

aci:AssetImpairmentChargesAndMineClosureCosts

aci:PriorEquityInvestment


Asset
Period End2020-12-31
Value10.0
Percentage of AssetImpairmentChargesAndMineClosureCosts100.0

aci:AssetImpairmentChargesAndMineClosureCosts us-gaap:StatementBusinessSegments, us-gaap:Subsegments

aci:PowderRiverBasin, aci:CoalCreekMine


Asset
Period End2020-09-30
Value51
Percentage of AssetImpairmentChargesAndMineClosureCosts0.0

us-gaap:AllOtherSegments, aci:ViperMine


Asset
Period End2020-09-30
Value33
Percentage of AssetImpairmentChargesAndMineClosureCosts0.0

us-gaap:AllOtherSegments, aci:WestElkMine


Asset
Period End2020-09-30
Value41
Percentage of AssetImpairmentChargesAndMineClosureCosts0.0

aci:AssetImpairmentChargesAndMineClosureCosts us-gaap:Subsegments

aci:CoalCreekMine


Asset
Period End2020-12-31
Value32
Percentage of AssetImpairmentChargesAndMineClosureCosts328.0

aci:BaseCappedCallTransactionCapPrice us-gaap:DebtInstrument

aci:BaseCappedCallTransactionCapPrice

aci:CappedCallTransactions


Base
Period End2020-10-29
Value52.255
Percentage of BaseCappedCallTransactionCapPrice100.0

aci:BaseCappedCallTransactionInitialStrikePrice us-gaap:DebtInstrument

aci:BaseCappedCallTransactionInitialStrikePrice

aci:CappedCallTransactions


Base
Period End2020-10-29
Value37.325
Percentage of BaseCappedCallTransactionInitialStrikePrice100.0

aci:BaseCappedCallTransactionNumberOfUnderlyingShares us-gaap:DebtInstrument

aci:BaseCappedCallTransactionNumberOfUnderlyingShares

aci:CappedCallTransactions


Base
Period End2020-10-29
Value4200000.0
Percentage of BaseCappedCallTransactionNumberOfUnderlyingShares100.0

aci:BaseCappedCallTransactionPremiumPercentage us-gaap:DebtInstrument

aci:BaseCappedCallTransactionPremiumPercentage

aci:CappedCallTransactions


Base
Period End2020-10-29
Value75.0
Percentage of BaseCappedCallTransactionPremiumPercentage100.0

aci:ChangeInFairValueOfCoalDerivativesAndCoalTradingActivitiesNet

aci:ChangeInFairValueOfCoalDerivativesAndCoalTradingActivitiesNet


Change
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value8.7620.5281.9562.649-0.1290.7431.251.53-8.4-12.981-13.0241015-3.4144.4771.0280.8630.8540.3960.4881.1581.21-0.455-3.5591.2111.222.125-3.733-2.9920.9145.7929.753-9.0081.308135.84-32.054-3.613-12.1558.362.672-1.784-3.3721.8324.5875.877
Percentage of ChangeInFairValueOfCoalDerivativesAndCoalTradingActivitiesNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:DebtInstrumentCovenantCrossEventsOfDefaultToIndebtedness us-gaap:Range, us-gaap:DebtInstrument

aci:DebtInstrumentCovenantCrossEventsOfDefaultToIndebtedness

srt:Minimum, aci:TaxExemptBonds


Debt
Period End2020-09-30
Value50
Percentage of DebtInstrumentCovenantCrossEventsOfDefaultToIndebtedness100.0

aci:DividendEquivalentsEarnedEquityInstrumentsOtherThanOptionsGrants

aci:DividendEquivalentsEarnedEquityInstrumentsOtherThanOptionsGrants


Dividend
Period End2020-09-302020-06-30
Value10000.059000.0
Percentage of DividendEquivalentsEarnedEquityInstrumentsOtherThanOptionsGrants100.0100.0

aci:DividendEquivalentsEarnedEquityInstrumentsOtherThanOptionsGrants us-gaap:StatementEquityComponents

us-gaap:RetainedEarnings


Dividend
Period End2020-09-302020-06-30
Value10000.059000.0
Percentage of DividendEquivalentsEarnedEquityInstrumentsOtherThanOptionsGrants100.0100.0

aci:EBITDFromIdledOrOtherwiseDisposedOperations

aci:EBITDFromIdledOrOtherwiseDisposedOperations


E
Period End2021-06-302021-03-312020-06-302020-03-31
Value3.9973.5662.6965.099
Percentage of EBITDFromIdledOrOtherwiseDisposedOperations100.0100.0100.0100.0

aci:EffectiveIncomeTaxRateReconciliationAmtRefundAmount

aci:EffectiveIncomeTaxRateReconciliationAmtRefundAmount


Effective
Period End2020-09-302020-06-302020-03-31
Value00-1171000.0
Percentage of EffectiveIncomeTaxRateReconciliationAmtRefundAmount0.00.0100.0

aci:EquityMethodInvestmentPercentOfOwnershipInterestAcquired us-gaap:Ownership

aci:EquityMethodInvestmentPercentOfOwnershipInterestAcquired

aci:KnightHawk


Equity
Period End2020-12-31
Value1.5
Percentage of EquityMethodInvestmentPercentOfOwnershipInterestAcquired100.0

aci:FinanceLeaseCost

aci:FinanceLeaseCost


Finance
Period End2021-06-302021-03-312020-12-312020-09-302020-06-30
Value393000.0393000.0786000.00393000.0
Percentage of FinanceLeaseCost100.0100.0100.00.0100.0

aci:ImpairmentOfCoalLandsAndMineralRights

aci:ImpairmentOfCoalLandsAndMineralRights


Impairment
Period End2020-09-30
Value33
Percentage of ImpairmentOfCoalLandsAndMineralRights100.0

aci:ImpairmentOfDeferredDevelopmentCosts

aci:ImpairmentOfDeferredDevelopmentCosts


Impairment
Period End2020-09-30
Value50
Percentage of ImpairmentOfDeferredDevelopmentCosts100.0

aci:IncomeTaxRefundReservePercentageOfTotalRefundSubjectToAudit

aci:IncomeTaxRefundReservePercentageOfTotalRefundSubjectToAudit


Income
Period End2020-09-30
Value40.0
Percentage of IncomeTaxRefundReservePercentageOfTotalRefundSubjectToAudit100.0

aci:LineOfCreditLiquidityCovenantAmount us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType

aci:LineOfCreditLiquidityCovenantAmount

us-gaap:LineOfCredit, aci:InventoryBasedRevolvingCreditFacility, aci:RegionsBank, us-gaap:SecuredDebt


Line
Period End2020-09-302020-08-31
Value100175
Percentage of LineOfCreditLiquidityCovenantAmount100.0100.0

aci:LineOfCreditLiquidityCovenantAmount us-gaap:DebtInstrument

aci:InventoryBasedRevolvingCreditFacility


Line
Period End2021-06-302021-03-31
Value5.912
Percentage of LineOfCreditLiquidityCovenantAmount0.00.0

aci:OperatingCostExpensesAndOthers

aci:OperatingCostExpensesAndOthers


Operating
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value417357433569364429556512503478
Percentage of OperatingCostExpensesAndOthers100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:OperatingIncomeExpenseRelatedToJointventure

aci:OperatingIncomeExpenseRelatedToJointventure


Operating
Period End2020-09-302020-06-302020-03-312019-09-302019-06-30
Value4.4237.8513.6643.7543.018
Percentage of OperatingIncomeExpenseRelatedToJointventure100.0100.0100.0100.0100.0

aci:OtherIncomeExpense

aci:OtherIncomeExpense


Other
Period End2021-06-302021-03-312020-06-302020-03-31
Value8376000.0-1265000.0-906000.059000.0
Percentage of OtherIncomeExpense100.0100.0100.0100.0

aci:ProceedsFromConsiderationPaidForDisposalsAndDivestitures

aci:ProceedsFromConsiderationPaidForDisposalsAndDivestitures


Proceeds
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-31
Value250000.0188000.0294000.0329000.0233000.0208000.0983000.0608000.0
Percentage of ProceedsFromConsiderationPaidForDisposalsAndDivestitures100.0100.0100.0100.0100.0100.0100.0100.0

aci:ProceedsFromEquipmentFinancing

aci:ProceedsFromEquipmentFinancing


Proceeds
Period End2020-12-312020-09-302020-06-302020-03-31
Value0.00.00.053
Percentage of ProceedsFromEquipmentFinancing0.00.00.0100.0

aci:PurchasesOfInvestmentsAdvancesToAffiliates

aci:PurchasesOfInvestmentsAdvancesToAffiliates


Purchases
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value0.01.1140.00.490.320.7390.6891.5351.0792.1960.6641.8170.00.00.9570.2821.0297.9050.8230.550.7342.1563.5582.8063.2951.8433.2643.8926.2593.2424.1362.9823.8444.2985.0733.3933.5155.7775.082183.6434292.4914.17810
Percentage of PurchasesOfInvestmentsAdvancesToAffiliates0.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:RealizedAndUnrealizedGainLossOnDerivatives us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation

aci:RealizedAndUnrealizedGainLossOnDerivatives

srt:NaturalGasReserves, us-gaap:Nondesignated, aci:ChangeInFairValueOfCoalDerivativesAndCoalTradingActivityNetDomain


Realized
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value00076000.0-33000.014000.0-19000.0-61000.0
Percentage of RealizedAndUnrealizedGainLossOnDerivatives0.00.00.0100.0100.0100.0100.0100.0

us-gaap:CoalContract, us-gaap:Nondesignated, aci:ChangeInFairValueOfCoalDerivativesAndCoalTradingActivityNetDomain


Realized
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value01000.00221000.0215000.0-127000.0-718000.0-383000.0
Percentage of RealizedAndUnrealizedGainLossOnDerivatives0.00.00.0290.8-651.5-907.13778.9627.9

aci:SegmentAdjustedEBITDFromCoalOperations

aci:SegmentAdjustedEBITDFromCoalOperations


Segment
Period End2021-06-302021-03-312020-06-302020-03-31
Value103541040
Percentage of SegmentAdjustedEBITDFromCoalOperations100.0100.0100.0100.0

aci:TotalWorkersCompensationExpense aci:DefinedBenefitPlansBenefitObligationExpenses

aci:TotalWorkersCompensationExpense

aci:WorkersCompensation


Total
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-312010-09-302010-06-30
Value5.5155.4559.7165.6175.6885.7698.9375.1125.4335.1674.9050.9875.2426.0662.1711.1445.2595.6055.8716.0216.2646.1888.7549.958.5017.9757.1017.4356.5243.7142.5862.5282.264
Percentage of TotalWorkersCompensationExpense100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:ValueOfTradingPortfolioRealizedGainLoss

aci:ValueOfTradingPortfolioRealizedGainLoss


Value
Period End2020-12-31
Value-200000.0
Percentage of ValueOfTradingPortfolioRealizedGainLoss100.0

us-gaap:AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts

us-gaap:AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts


Adjustments
Period End2021-06-302021-03-31
Value01500000.0
Percentage of AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts0.0100.0

us-gaap:AmortizationOfDebtDiscountPremium us-gaap:DebtInstrument

us-gaap:AmortizationOfDebtDiscountPremium

aci:ConvertibleSeniorNotesDue2025


Amortization
Period End2021-06-302021-03-312020-12-31
Value1800000.01700000.01000000.0
Percentage of AmortizationOfDebtDiscountPremium100.0100.0100.0

us-gaap:AmortizationOfFinancingCostsAndDiscounts

us-gaap:AmortizationOfFinancingCostsAndDiscounts


Amortization
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-30
Value1.7841.3262.411.2430.9750.9710.9340.9310.9190.9070.8791.131.091.081.1081.0631.030.5350.4675.1434.5073.156.2816.4216.326.2195.0144.5924.5213.2366.2646.179
Percentage of AmortizationOfFinancingCostsAndDiscounts100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AssetImpairmentCharges

us-gaap:AssetImpairmentCharges


Asset
Period End2020-09-302016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302013-12-312013-06-302012-12-31
Value1630.041772119170.00.01.51226520214
Percentage of AssetImpairmentCharges100.00.0100.0100.0100.0100.00.00.0100.0100.0100.0100.0

us-gaap:AssetImpairmentCharges us-gaap:StatementBusinessSegments, us-gaap:Subsegments

aci:PowderRiverBasin, aci:CoalCreekMine


Asset
Period End2020-09-30
Value51
Percentage of AssetImpairmentCharges31.8

us-gaap:AllOtherSegments, aci:ViperMine


Asset
Period End2020-09-30
Value33
Percentage of AssetImpairmentCharges20.5

us-gaap:AllOtherSegments, aci:WestElkMine


Asset
Period End2020-09-30
Value41
Percentage of AssetImpairmentCharges25.5

us-gaap:AssetRetirementObligationAccretionExpense

us-gaap:AssetRetirementObligationAccretionExpense


Asset
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value5.4375.4374.9484.9474.9865.0065.1375.1375.1375.1376.9936.9926.9936.9927.3837.587.6237.6237.634
Percentage of AssetRetirementObligationAccretionExpense100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AssetRetirementObligationAccretionExpense us-gaap:ConsolidationItems

us-gaap:IntersegmentElimination


Asset
Period End2021-06-302020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-31
Value0.5110.6190.6580.6780.8680.8680.8681.0731.0741.0731.4891.5151.515
Percentage of AssetRetirementObligationAccretionExpense9.412.513.213.516.916.916.915.315.415.319.619.919.9

us-gaap:AssetRetirementObligationAccretionExpense us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments

us-gaap:OperatingSegments, aci:MetallurgicalSegment


Asset
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value0.5074.4190.4850.4860.4860.4860.5310.5310.5310.5310.4680.4690.4690.4690.4320.5110.5280.5280.528
Percentage of AssetRetirementObligationAccretionExpense9.381.39.89.89.79.710.310.310.310.36.76.76.76.75.96.76.96.96.9

us-gaap:OperatingSegments, aci:PowderRiverBasin


Asset
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312016-12-31
Value0.5083.4953.4953.4953.4953.1363.1353.1353.1354.8854.8854.8854.8855.049
Percentage of AssetRetirementObligationAccretionExpense9.370.670.670.169.861.061.061.061.069.969.969.969.966.1

us-gaap:OperatingSegments, aci:ThermalSegment


Asset
Period End2021-06-302020-06-30
Value4.4193.842
Percentage of AssetRetirementObligationAccretionExpense81.377.1

us-gaap:OperatingSegments, us-gaap:AllOtherSegments


Asset
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value0.510.3470.3470.3470.3470.6030.6030.6030.6030.5650.5650.5650.5650.540.540.540.540.54
Percentage of AssetRetirementObligationAccretionExpense9.47.07.07.06.911.711.711.711.78.18.18.18.17.37.17.17.17.1

us-gaap:BankruptcyClaimsAmountOfClaimsFiled

us-gaap:BankruptcyClaimsAmountOfClaimsFiled


Bankruptcy
Period End2020-09-30
Value4300000.0
Percentage of BankruptcyClaimsAmountOfClaimsFiled100.0

us-gaap:DebtConversionOriginalDebtAmount1

us-gaap:DebtConversionOriginalDebtAmount1


Debt
Period End2021-03-31
Value114
Percentage of DebtConversionOriginalDebtAmount1100.0

us-gaap:DebtInstrumentConvertibleConversionRatio1 us-gaap:DebtInstrument

us-gaap:DebtInstrumentConvertibleConversionRatio1

aci:ConvertibleSeniorNotesDue2025


Debt
Period End2020-11-03
Value26.792
Percentage of DebtInstrumentConvertibleConversionRatio1100.0

us-gaap:DebtInstrumentRedemptionPricePercentage us-gaap:DebtInstrument

us-gaap:DebtInstrumentRedemptionPricePercentage

aci:TaxExemptBonds


Debt
Period End2020-07-02
Value100.0
Percentage of DebtInstrumentRedemptionPricePercentage100.0

aci:TaxExemptBonds2020


Debt
Period End2020-07-02
Value100.0
Percentage of DebtInstrumentRedemptionPricePercentage100.0

us-gaap:DefinedBenefitPlanAmortizationOfGainsLosses aci:DefinedBenefitPlansBenefitObligationExpenses

us-gaap:DefinedBenefitPlanAmortizationOfGainsLosses

aci:OccupationalDisease


Defined
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-06-302019-03-31
Value-590000.0-591000.0-297000.0-298000.0-297000.000
Percentage of DefinedBenefitPlanAmortizationOfGainsLosses100.0100.0100.0100.0100.00.00.0

us-gaap:DefinedBenefitPlanAmortizationOfGainsLosses us-gaap:RetirementPlanType

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302016-12-31
Value160000.0160000.0529000.0530000.0738000.02236000.000000
Percentage of DefinedBenefitPlanAmortizationOfGainsLosses0.0-53.9-177.5-178.50.00.00.00.00.00.00.0

us-gaap:DefinedBenefitPlanAmortizationOfPriorServiceCostCredit us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:DefinedBenefitPlanAmortizationOfPriorServiceCostCredit

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-31
Value43000.044000.028000.030000.027000.027000.0
Percentage of DefinedBenefitPlanAmortizationOfPriorServiceCostCredit100.0100.0100.0100.0100.0100.0

us-gaap:DefinedBenefitPlanAmortizationOfPriorServiceCostCredit us-gaap:RetirementPlanType

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-09-302016-12-31
Value-43000.0-28000.0-29000.0-27000.0-27000.0-28000.0-28000.04000.00000
Percentage of DefinedBenefitPlanAmortizationOfPriorServiceCostCredit-100.0-63.6-103.6-90.0-100.0-103.70.00.00.00.00.00.0

us-gaap:DefinedBenefitPlanExpectedReturnOnPlanAssets us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanExpectedReturnOnPlanAssets

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value1.8241.8241.952.0222.0232.2882.4092.6992.7242.7233.0162.9063.083.0813.5893.9134.4994.4974.77
Percentage of DefinedBenefitPlanExpectedReturnOnPlanAssets100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DefinedBenefitPlanInterestCost aci:DefinedBenefitPlansBenefitObligationExpenses

us-gaap:DefinedBenefitPlanInterestCost

aci:OccupationalDisease


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-31
Value1.1111.110.9191.3981.3991.3990.861.3541.3541.3540.781.1961.1941.1951.1451.1691.1681.169
Percentage of DefinedBenefitPlanInterestCost100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DefinedBenefitPlanInterestCost us-gaap:RetirementPlanType

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value0.5280.5280.5430.5430.6530.6530.8760.8760.8770.8760.9190.9180.9190.9180.9751.0581.0591.0580.978
Percentage of DefinedBenefitPlanInterestCost47.547.659.138.846.746.7101.964.764.864.7117.876.877.076.885.290.590.790.50.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value1.0671.0681.1561.2551.4831.6041.7021.9222.2592.2582.3522.3762.272.2712.4512.7362.9912.9912.768
Percentage of DefinedBenefitPlanInterestCost96.096.2125.889.8106.0114.7197.9141.9166.8166.8301.5198.7190.1190.0214.1234.0256.1255.90.0

us-gaap:DefinedBenefitPlanNetPeriodicBenefitCost aci:DefinedBenefitPlansBenefitObligationExpenses

us-gaap:DefinedBenefitPlanNetPeriodicBenefitCost

aci:OccupationalDisease


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value3.653.653.1063.5873.5883.5872.5293.0243.0233.0232.643.0563.0543.0557.45-1.9332.7272.7272.709
Percentage of DefinedBenefitPlanNetPeriodicBenefitCost100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DefinedBenefitPlanNetPeriodicBenefitCost us-gaap:RetirementPlanType

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value0.6130.6130.4860.4870.230.2290.258-1.240.9970.9961.0581.0581.0581.0581.1350.7081.231.2281.158
Percentage of DefinedBenefitPlanNetPeriodicBenefitCost16.816.815.613.66.46.410.2-41.033.032.940.134.634.634.615.2-36.645.145.042.7

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value-1.788-0.784-1.152-1.231-0.701-0.708-1.446-0.97-0.894-0.465-1.012-1.143-2.181-0.81-1.954-1.406-1.995-1.506-2.137
Percentage of DefinedBenefitPlanNetPeriodicBenefitCost-49.0-21.5-37.1-34.3-19.5-19.7-57.2-32.1-29.6-15.4-38.3-37.4-71.4-26.5-26.272.7-73.2-55.2-78.9

us-gaap:DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Defined
Period End2021-06-302020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302018-12-312018-09-302018-06-30
Value988000.0329000.0437000.0134000.0-4000.0700000.0197000.0429000.0348000.0613000.01371000.0
Percentage of DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 us-gaap:RetirementPlanType

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2021-06-302020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-312017-09-302017-06-302016-12-31
Value988000.0329000.0437000.0134000.0-4000.0700000.0197000.0429000.00348000.0613000.01371000.0816000.0229000.0487000.0135000.0
Percentage of DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.00.00.00.00.0

us-gaap:DefinedBenefitPlanServiceCost aci:DefinedBenefitPlansBenefitObligationExpenses

us-gaap:DefinedBenefitPlanServiceCost

aci:OccupationalDisease


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-31
Value1.9491.9491.891.8921.8911.8911.6691.671.6691.6691.861.861.861.861.6451.5581.5591.558
Percentage of DefinedBenefitPlanServiceCost100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DefinedBenefitPlanServiceCost us-gaap:RetirementPlanType

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value0.0850.0850.1030.1040.1060.1060.120.120.120.120.1390.140.1390.140.160.170.1710.170.18
Percentage of DefinedBenefitPlanServiceCost4.44.45.45.55.65.67.27.27.27.27.57.57.57.59.710.911.010.90.0

us-gaap:DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation

us-gaap:DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet

aci:HeatingOilDieselPurchases, us-gaap:Nondesignated, us-gaap:OtherOperatingIncomeExpense


Derivative
Period End2020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-09-30
Value-0.1840.328-1.0330.111-1.67-1.3690.637-4.8990.7193.6570.0182.8460.822-1.147-3.5780.827-0.772.039-0.443-5.5250.628-2.365-3.7460.0-2.963-0.288
Percentage of DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0

us-gaap:DerivativeNonmonetaryNotionalAmountMass aci:ReportingYear, us-gaap:Position

us-gaap:DerivativeNonmonetaryNotionalAmountMass

aci:YearOne, us-gaap:Long


Derivative
Period End2021-06-302021-03-312020-09-30
Value59000.062000.0-139000.0
Percentage of DerivativeNonmonetaryNotionalAmountMass100.0100.0100.0

aci:YearOne, us-gaap:Short


Derivative
Period End2021-06-302021-03-312020-09-30
Value215000.0320000.0-287000.0
Percentage of DerivativeNonmonetaryNotionalAmountMass364.4516.1206.5

us-gaap:DerivativeNonmonetaryNotionalAmountMass us-gaap:Position

us-gaap:Long


Derivative
Period End2020-09-302019-09-302019-06-302019-03-312018-09-302018-06-302018-03-31
Value-139000.0-275000.0-227000.01224000.0-110000.0-400000.0912000.0
Percentage of DerivativeNonmonetaryNotionalAmountMass100.00.00.00.00.00.00.0

us-gaap:Short


Derivative
Period End2020-09-302019-09-302019-06-302019-03-312018-09-302018-06-302018-03-31
Value-254000.0-754000.0-511000.02540000.0-410000.0-49000.02725000.0
Percentage of DerivativeNonmonetaryNotionalAmountMass182.70.00.00.00.00.00.0

us-gaap:DerivativeNonmonetaryNotionalAmountVolume us-gaap:ConcentrationRiskByType, us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation

us-gaap:DerivativeNonmonetaryNotionalAmountVolume

aci:DieselPurchases, aci:NYMEX, us-gaap:Nondesignated


Derivative
Period End2020-09-302020-06-302020-03-312019-09-302019-06-302019-03-31
Value-2.00.04.0-6.0-6.018
Percentage of DerivativeNonmonetaryNotionalAmountVolume100.00.0100.0100.0100.0100.0

us-gaap:DerivativeNonmonetaryNotionalAmountVolume us-gaap:HedgingDesignation, us-gaap:TradingActivityByType

us-gaap:Nondesignated, srt:HeatingOil


Derivative
Period End2020-09-302020-06-302020-03-312019-09-30
Value-4.0-9.015-6.0
Percentage of DerivativeNonmonetaryNotionalAmountVolume200.00.0375.0100.0

us-gaap:DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax

us-gaap:DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax


Discontinued
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312013-12-312013-09-30
Value0.2250.1880.2670.282.9660.214-9.122-0.5970.940.4750.5710.1850.003-0.1341.321211.6580.3470.4854.642115
Percentage of DiscontinuedOperationGainLossFromDisposalOfDiscontinuedOperationBeforeIncomeTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:EquityMethodInvestmentOtherThanTemporaryImpairment

us-gaap:EquityMethodInvestmentOtherThanTemporaryImpairment


Equity
Period End2020-09-302013-12-312013-09-30
Value360.013
Percentage of EquityMethodInvestmentOtherThanTemporaryImpairment100.00.0100.0

us-gaap:EquityMethodInvestmentOtherThanTemporaryImpairment us-gaap:ScheduleOfEquityMethodInvestmentEquityMethodInvesteeName

aci:KnightHawk


Equity
Period End2020-12-312020-09-30
Value-0.03336
Percentage of EquityMethodInvestmentOtherThanTemporaryImpairment0.0100.1

us-gaap:FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss us-gaap:FairValueByFairValueHierarchyLevel

us-gaap:FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss

us-gaap:FairValueInputsLevel3


Fair
Period End2020-12-312020-09-302020-06-302019-09-302019-06-302018-09-302018-06-302017-09-302017-06-302016-09-302016-06-302015-09-302015-06-302014-09-302014-06-302013-09-30
Value00-100000.0-1000000.0-1400000.0800000.01500000.0700000.0-1500000.0-500000.01400000.0-3900000.0200000.0-1800000.0500000.0200000.0
Percentage of FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss0.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1

us-gaap:FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1


Fair
Period End2020-12-312020-09-302020-06-302019-12-312019-09-302019-06-302018-12-312018-09-302018-06-302017-12-312017-09-302017-06-302016-09-302016-06-302015-12-312015-09-302015-06-302014-12-312014-09-302014-06-302013-12-312013-09-302013-06-302012-12-312012-09-302012-06-30
Value2316000.0-33000.0-56000.03687000.0-1252000.0-398000.04704000.0-749000.0-3628000.0-2451000.0-644000.01332000.0-1888000.01061000.0-2574000.0-4200000.0-481000.0-1721000.0-2131000.0205000.0-371000.0-570000.0-4840000.0-6068000.04265000.0-14092000.0
Percentage of FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues

us-gaap:FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues


Fair
Period End2020-12-312019-12-312019-09-302019-06-302018-12-312018-09-302018-06-302017-12-312017-09-302017-06-302016-09-302016-06-302015-12-312015-09-302015-06-302014-12-312014-09-302013-12-312013-09-302011-06-30
Value0.00.1630.0450.00.00.070.580.020.0160.3170.00.2551.8320.5580.0-0.175-0.4470.00.0-2.16
Percentage of FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues0.0100.0100.00.00.0100.0100.0100.0100.0100.00.0100.0100.0100.00.0100.0100.00.00.0100.0

us-gaap:FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases

us-gaap:FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases


Fair
Period End2020-12-312019-12-312019-09-302019-06-302018-12-312018-09-302018-06-302017-12-312017-09-302017-06-302016-09-302016-06-302015-12-312015-09-302015-06-302014-12-312014-09-302014-06-302013-12-312013-09-302013-06-302012-12-312012-09-302012-06-302011-09-302011-06-30
Value0.00.0350.5620.5370.9390.8250.8231.4660.8911.1581.5862.0731.5822.3341.8420.3971.081.8551.3371.4642.636-14.5982.8696.4689.9359.006
Percentage of FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases0.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:FinanceLeaseInterestPaymentOnLiability

us-gaap:FinanceLeaseInterestPaymentOnLiability


Finance
Period End2021-06-302021-03-312020-12-312020-09-302020-06-30
Value303000.0303000.0606000.00303000.0
Percentage of FinanceLeaseInterestPaymentOnLiability100.0100.0100.00.0100.0

us-gaap:GainLossOnSaleOfDerivatives us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation

us-gaap:GainLossOnSaleOfDerivatives

us-gaap:CoalContract, us-gaap:Nondesignated, us-gaap:OtherOperatingIncomeExpense


Gain
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-09-30
Value-0.6510.1382.3082.5932.7561.6012.5633.216-0.881-4.411-3.517-2.537-1.649-1.0310.00.00.0-0.133-0.180.1630.5110.8261.0910.5021.3182.8799.929
Percentage of GainLossOnSaleOfDerivatives100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.00.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:GainLossRelatedToLitigationSettlement

us-gaap:GainLossRelatedToLitigationSettlement


Gain
Period End2020-09-302020-06-302020-03-312019-12-312019-09-302018-12-312018-09-302012-12-312012-09-30
Value0.0149.00.0390.00.0-79.53279
Percentage of GainLossRelatedToLitigationSettlement0.0100.0100.00.0100.00.00.0100.0100.0

us-gaap:GainLossRelatedToLitigationSettlement us-gaap:GainContingenciesByNature

aci:PreferenceRightsLeaseApplication


Gain
Period End2020-12-312020-09-302019-09-30
Value108.739
Percentage of GainLossRelatedToLitigationSettlement0.00.0100.0

us-gaap:GainOnBusinessInterruptionInsuranceRecovery

us-gaap:GainOnBusinessInterruptionInsuranceRecovery


Gain
Period End2020-12-312020-09-302020-06-302020-03-31
Value0.00.0149.0
Percentage of GainOnBusinessInterruptionInsuranceRecovery0.00.0100.0100.0

us-gaap:GainOnBusinessInterruptionInsuranceRecovery us-gaap:BusinessInterruptionLoss

aci:MountainLaurelLongwallOperations


Gain
Period End2020-09-302020-06-302020-03-31
Value0.0149.0
Percentage of GainOnBusinessInterruptionInsuranceRecovery0.099.9100.0

us-gaap:ImpairmentOfLongLivedAssetsHeldForUse

us-gaap:ImpairmentOfLongLivedAssetsHeldForUse


Impairment
Period End2020-09-302013-09-30
Value43142
Percentage of ImpairmentOfLongLivedAssetsHeldForUse100.0100.0

us-gaap:IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance

us-gaap:IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance


Income
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value-3.2420.4121540190.8518-19.12-11.292-12.513-17.727-44.278-7.317-10.639-371.647-19.118-8.316-11.9-7.655
Percentage of IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate

us-gaap:IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate


Income
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value6.273-1.189-16.449-40.128-10.105-5.689-1.8522131517168.387121623131812
Percentage of IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:IncomeTaxReconciliationNondeductibleExpense

us-gaap:IncomeTaxReconciliationNondeductibleExpense


Income
Period End2021-06-302020-06-30
Value-4038000.0-11768000.0
Percentage of IncomeTaxReconciliationNondeductibleExpense100.0100.0

us-gaap:IncomeTaxReconciliationNondeductibleExpenseDepletion

us-gaap:IncomeTaxReconciliationNondeductibleExpenseDepletion


Income
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value1660000.0-4405000.02242000.0-11768000.06168000.0-3632000.0-6347000.0-3457000.0-4307000.0-4666000.0-5964000.0-2488000.0-4607000.0-10816000.0-7708000.0-5692000.0-7039000.0-4292000.0
Percentage of IncomeTaxReconciliationNondeductibleExpenseDepletion100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:IncomeTaxReconciliationOtherAdjustments

us-gaap:IncomeTaxReconciliationOtherAdjustments


Income
Period End2021-06-302021-03-312020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-31
Value-15000.0-574000.0000-51000.0-5000.03393000.0-2531000.01465000.0-19000.0801000.0920000.0
Percentage of IncomeTaxReconciliationOtherAdjustments100.0100.00.00.00.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:IncomeTaxReconciliationStateAndLocalIncomeTaxes

us-gaap:IncomeTaxReconciliationStateAndLocalIncomeTaxes


Income
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value1180000.0-292000.0899000.0-2695000.0-1584000.082000.0-16437000.01825000.0831000.01012000.0-1385000.04528000.0583000.0754000.05383000.0728000.0408000.0483000.0633000.0
Percentage of IncomeTaxReconciliationStateAndLocalIncomeTaxes100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:IncomeTaxReconciliationTaxSettlements

us-gaap:IncomeTaxReconciliationTaxSettlements


Income
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-30
Value1848000.0361000.0226000.05132000.0-2031000.01495000.0844000.0593000.0511000.0792000.01389000.01127000.0
Percentage of IncomeTaxReconciliationTaxSettlements100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:IncreaseDecreaseInDeferredIncomeTaxes

us-gaap:IncreaseDecreaseInDeferredIncomeTaxes


Increase
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value0.361-0.372-0.203-0.347-14.4850.6050.179-0.295-13.3850.0-41.7313.397-12.12728-0.073-0.166-5.83-0.0030.001-0.0110.429203394.0773.433-138.15343443447-286.59475480-16.5642502113-2.699-21.8991.0264.8442.8424.869-0.15
Percentage of IncreaseDecreaseInDeferredIncomeTaxes100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:IncreaseDecreaseInDerivativeAssetsAndLiabilities

us-gaap:IncreaseDecreaseInDerivativeAssetsAndLiabilities


Increase
Period End2021-03-312020-03-312016-12-312013-12-312012-12-31
Value0.0331.073-1.6622.038-42.681
Percentage of IncreaseDecreaseInDerivativeAssetsAndLiabilities100.0100.0100.0100.0100.0

us-gaap:IncreaseDecreaseInIncomeTaxesPayableNetOfIncomeTaxesReceivable

us-gaap:IncreaseDecreaseInIncomeTaxesPayableNetOfIncomeTaxesReceivable


Increase
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-30
Value-11.49130.004-0.06323-1.0735.7127.865240.076-45.029-13.502-0.036111.3661.169-4.965-3.28-0.295-0.642-4.266-0.0770.040.057-0.456-0.115-5.3070.816-0.4870.4585.497-0.821-0.133230.212-6.785
Percentage of IncreaseDecreaseInIncomeTaxesPayableNetOfIncomeTaxesReceivable100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:IncreaseDecreaseInOtherAccountsPayableAndAccruedLiabilities

us-gaap:IncreaseDecreaseInOtherAccountsPayableAndAccruedLiabilities


Increase
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value4.408-7.585-3.688-39.201-29.791-2.92822-17.86110-12.88314-26.052125.19220-20.529346.7644393.80637-63.6057.186-31.64462-33.49443-96.80823-42.31138-119.612121.257-66.22248-5.1740-31.59666-11.18138-6.844
Percentage of IncreaseDecreaseInOtherAccountsPayableAndAccruedLiabilities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:IncreaseDecreaseInReceivables

us-gaap:IncreaseDecreaseInReceivables


Increase
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value3018-16.2418.401-32.089-23.728-35.33522-10.461-7.4116-14.229-8.51628-32.482040-37.134223515-7.815-101.377155.819-18.2522.2127.046-27.512279.555-76.345-8.43112-11.279-49.96135-88.0823910-28.25753-41.4314.6617.04437
Percentage of IncreaseDecreaseInReceivables100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:InterestExpenseDebtExcludingAmortization us-gaap:DebtInstrument

us-gaap:InterestExpenseDebtExcludingAmortization

aci:ConvertibleSeniorNotesDue2025


Interest
Period End2021-06-302021-03-312020-12-31
Value2000000.02000000.01300000.0
Percentage of InterestExpenseDebtExcludingAmortization100.0100.0100.0

us-gaap:InterestRateCashFlowHedgeGainLossReclassifiedToEarningsNet us-gaap:DerivativeInstrumentRisk

us-gaap:InterestRateCashFlowHedgeGainLossReclassifiedToEarningsNet

us-gaap:InterestRateSwap


Interest
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302018-12-312018-09-302018-06-30
Value-700000.0-600000.0-3600000.0600000.0600000.0-200000.0-100000.0200000.0500000.0400000.0300000.0300000.0
Percentage of InterestRateCashFlowHedgeGainLossReclassifiedToEarningsNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:NondebtorReorganizationItemsNetGainLossOnSettlementOfOtherClaims1

us-gaap:NondebtorReorganizationItemsNetGainLossOnSettlementOfOtherClaims1


Nondebtor
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-31
Value00026000.0-47000.00-16000.087000.0-60000.0-560000.0-740000.0-301000.0-43000.0-21000.0-2828000.0-20904000.0-21271000.0-3875000.0000
Percentage of NondebtorReorganizationItemsNetGainLossOnSettlementOfOtherClaims10.00.00.0100.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.00.00.0

us-gaap:OperatingLeaseCost

us-gaap:OperatingLeaseCost


Operating
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value846000.0846000.02774000.0-59000.0905000.01023000.02850000.0154000.00917000.0
Percentage of OperatingLeaseCost100.0100.0100.0100.0100.0100.0100.0100.00.0100.0

us-gaap:OperatingLeasePayments

us-gaap:OperatingLeasePayments


Operating
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value826000.0860000.02750000.0-25000.0885000.01042000.02764000.0207000.00871000.0
Percentage of OperatingLeasePayments100.0100.0100.0100.0100.0100.0100.0100.00.0100.0

us-gaap:OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentBeforeTaxPortionAttributableToParent

us-gaap:OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentBeforeTaxPortionAttributableToParent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-09-302013-06-302013-03-312012-06-302012-03-31
Value-16000.0101000.0-141000.0-255000.0533000.0-467000.0-230000.0-98000.0274000.0377000.036000.0102000.0177000.0-658000.000-387000.0-438000.0504000.02903000.0-362000.068000.0291000.0-2401000.0-1203000.0-2033000.01136000.04959000.01553000.097000.0394000.0
Percentage of OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentBeforeTaxPortionAttributableToParent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent

us-gaap:OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-09-302013-06-30
Value-16000.0101000.0-141000.0-255000.0533000.0-467000.0-230000.0-98000.0274000.0377000.036000.0102000.0177000.0-658000.000-387000.0-438000.0504000.01860000.0-234000.040000.0187000.0-1537000.0-770000.0-1301000.0688000.03171000.0
Percentage of OtherComprehensiveIncomeAvailableforsaleSecuritiesAdjustmentNetOfTaxPortionAttributableToParent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentBeforeTaxPortionAttributableToParent

us-gaap:OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentBeforeTaxPortionAttributableToParent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-09-302013-06-302013-03-312012-06-302012-03-31
Value-3.963-0.54727-4.1642.33614304.019-2.9020.0-14.202-2.736-3.6530.08.521-3.1540.0-2.2432.7491.338-1.182-3.474-0.2941.212.2691.847-1.2771.751.954-5.60.724
Percentage of OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentBeforeTaxPortionAttributableToParent100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.00.0100.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent

us-gaap:OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-09-302013-06-30
Value-3.963-0.54727-4.1642.33614304.019-2.9020.0-14.202-2.736-3.6530.08.521-3.1540.0-2.2432.7490.857-0.757-2.222-0.1890.7751.4521.182-0.819-1.12
Percentage of OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPortionAttributableToParent100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.00.0100.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax

us-gaap:OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax


Other
Period End2021-03-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value0.1764.3-1.358-17.363-28.72-0.8853.0993.9719-1.036-6.7286.0392.5790.004-8.671-0.271-1.1431.932-21.483-0.725-0.748-1.6146
Percentage of OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax us-gaap:StatementEquityComponents

us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Other
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value-27.0724.492-1.943-14.01-29.437-1.5863.3310.0143.3495.0240.02.195-9.4253.6410.00.00.00.0-8.7230.00.00.0-22.5160.00.00.046
Percentage of OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax0.0104.5143.180.7102.5179.2107.50.072.9-323.3-74.70.085.10.00.00.00.00.00.0100.6-0.0-0.00.0104.8-0.0-0.0-0.099.5

us-gaap:AccumulatedGainLossNetCashFlowHedgeParent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-31
Value0.0060.065-0.037-0.065-0.089-2.885
Percentage of OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax0.036.90.0-1.56.616.6

us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value-0.0030.111-0.145-0.1270.674-0.468-0.095-0.0630.2890.3770.0360.110.193-0.6580.0550.00.0-0.0550.1940.5040.0030.068-0.956-0.842-1.603-0.952-1.629-1.489-1.6571.282
Percentage of OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax0.063.10.0-3.0-49.62.70.37.19.39.50.2-10.6-2.9-10.92.10.00.00.00.00.075.0-0.8352.873.7-83.04.4224.7199.1102.92.7

us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent

us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value637000.0689000.0502000.0476000.0363000.0-2669000.0-3471000.0-2360000.0-2778000.02717000.0
Percentage of OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTaxParent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationBeforeTaxParent

us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationBeforeTaxParent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value637000.0689000.0502000.0476000.0363000.0-2669000.0-3471000.0-2360000.0-2778000.02717000.0
Percentage of OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationBeforeTaxParent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation, us-gaap:Position

us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax

us-gaap:CoalContract, us-gaap:DesignatedAsHedgingInstrument, us-gaap:CostOfSales, us-gaap:Long


Other
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value0.0740.025-0.439-0.156-0.291-0.034-0.34-0.566
Percentage of OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:CoalContract, us-gaap:DesignatedAsHedgingInstrument, us-gaap:SalesRevenueNet, us-gaap:Short


Other
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value-0.074-0.0250.440.1590.7770.2254.015.237
Percentage of OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax-100.0-100.0-100.2-101.9-267.0-661.8-1179.4-925.3

us-gaap:OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax us-gaap:HedgingDesignation

us-gaap:DesignatedAsHedgingInstrument


Other
Period End2020-09-302020-06-302020-03-312019-09-302019-06-302019-03-31
Value0.00.0010.0030.1913.674.671
Percentage of OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax0.0-0.2-1.9-561.8-1079.4-825.3

us-gaap:OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent

us-gaap:OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-09-302015-06-302015-03-312014-09-302014-06-302014-03-312013-09-302012-03-312011-03-31
Value4.5841.337-26.9324.385-1.44-17.403-34.622-6.4770.3983.094222.902-8.4635.899-8.543.411-0.3681.656-2.4070.86-1.0940.2163.604-1.979-1.577-2.632.3677.38.697
Percentage of OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherLaborRelatedExpenses aci:DefinedBenefitPlansBenefitObligationExpenses

us-gaap:OtherLaborRelatedExpenses

aci:TraumaticInjuryClaimsAndAssessments


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value1.8651.8056.612.032.12.1826.4082.0882.412.144
Percentage of OtherLaborRelatedExpenses100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherNonoperatingIncome

us-gaap:OtherNonoperatingIncome


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-30
Value0.1470.3280.2970.4591.7931.2591.7513.7092.0882.1432.1561.8011.5521.2730.560.720.8420.5270.4870.5820.9331.1380.4230.6720.9622.3731.931.9492.0361.8431.8540.6971.2162.8361.9051.459
Percentage of OtherNonoperatingIncome100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherNonoperatingIncome us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-30
Value0.0140.01-0.0030.1280.141-0.0010.1350.0350.0150.00.00.0080.0160.00.0550.0
Percentage of OtherNonoperatingIncome9.53.0-1.027.97.9-0.17.70.90.70.00.00.41.00.09.80.0

us-gaap:OtherOperatingIncomeExpenseNet

us-gaap:OtherOperatingIncomeExpenseNet


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-30
Value4.3475.2685.4784.8945.7046.179.8694.2543.2391.65-0.7097.077.3186.932168.253.5182.315.342.476102.22-11.6468.625-7.403-9.086101.2210.2327.998-18.7925.3958.2393.217-25.386251.831181.9153.6134.7321.1164.727.2214.392
Percentage of OtherOperatingIncomeExpenseNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:PaymentsOfFinancingCosts

us-gaap:PaymentsOfFinancingCosts


Payments
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value0.3431.1946.192.3570.7490.4220.00.00.2480.480.5290.00.1061.1431.6727.2280.01.3750.403180.00.00.00.02.30.2620.01.957150.305340.10.2362.2531120.0080.121120.2370.2
Percentage of PaymentsOfFinancingCosts100.0100.0100.0100.0100.0100.00.00.0100.0100.0100.00.0100.0100.0100.0100.00.0100.0100.0100.00.00.00.00.0100.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:PaymentsOfOrdinaryDividends

us-gaap:PaymentsOfOrdinaryDividends


Payments
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302016-12-31
Value0.60.00.07.6457.9567.07.4257.8397.3037.6337.9988.3357.6068.20.0
Percentage of PaymentsOfOrdinaryDividends100.00.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.0

us-gaap:PaymentsRelatedToTaxWithholdingForShareBasedCompensation

us-gaap:PaymentsRelatedToTaxWithholdingForShareBasedCompensation


Payments
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-31
Value01316000.01849000.015000.0133000.0198000.0
Percentage of PaymentsRelatedToTaxWithholdingForShareBasedCompensation0.0100.0100.0100.0100.0100.0

us-gaap:PaymentsToAcquireAvailableForSaleSecuritiesDebt

us-gaap:PaymentsToAcquireAvailableForSaleSecuritiesDebt


Payments
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302016-12-31
Value44580.51117466961273.23129713867330.0
Percentage of PaymentsToAcquireAvailableForSaleSecuritiesDebt100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.0

us-gaap:PaymentsToAcquireRoyaltyInterestsInMiningProperties

us-gaap:PaymentsToAcquireRoyaltyInterestsInMiningProperties


Payments
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value1.0620.0620.0620.0621.0620.0620.0620.0621.0620.0630.0620.3980.0620.0620.2630.8224.1480.0630.0630.0880.1460.0710.0635.3990.1960.2133.7130.2632.750.5914.0460.7391.029.1424.0770.5580.3728.2623.8220.9234.297203.640.2490.12623
Percentage of PaymentsToAcquireRoyaltyInterestsInMiningProperties100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:PensionAndOtherPostretirementBenefitExpense

us-gaap:PensionAndOtherPostretirementBenefitExpense


Pension
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302016-12-31
Value539000.01527000.0808000.0878000.01102000.01096000.0-74000.0-975000.01336000.01766000.0996000.0971000.0-68000.01303000.0166000.0821000.0232000.0-32000.0
Percentage of PensionAndOtherPostretirementBenefitExpense100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:PostemploymentBenefitsPeriodExpense us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents

us-gaap:PostemploymentBenefitsPeriodExpense

us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Postemployment
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-09-30
Value-590000.0-591000.0-519000.0-138000.0232000.0234000.0-5256000.02236000.0000
Percentage of PostemploymentBenefitsPeriodExpense100.0100.0100.0100.0100.0100.0100.0100.00.00.00.0

us-gaap:ProceedsFromConvertibleDebt

us-gaap:ProceedsFromConvertibleDebt


Proceeds
Period End2021-03-31
Value155
Percentage of ProceedsFromConvertibleDebt100.0

us-gaap:ProceedsFromConvertibleDebt us-gaap:DebtInstrument

aci:ConvertibleSeniorNotesDue2025


Proceeds
Period End2020-11-03
Value132
Percentage of ProceedsFromConvertibleDebt0.0

us-gaap:ProceedsFromInsuranceSettlementInvestingActivities

us-gaap:ProceedsFromInsuranceSettlementInvestingActivities


Proceeds
Period End2020-12-312020-09-302020-06-302020-03-31
Value0.00.0167.353
Percentage of ProceedsFromInsuranceSettlementInvestingActivities0.00.0100.0100.0

us-gaap:ProceedsFromIssuanceOfDebt

us-gaap:ProceedsFromIssuanceOfDebt


Proceeds
Period End2021-06-302021-03-312020-12-31
Value0.0440.0
Percentage of ProceedsFromIssuanceOfDebt0.0100.00.0

us-gaap:ProceedsFromIssuanceOfSecuredDebt us-gaap:DebtInstrument

us-gaap:ProceedsFromIssuanceOfSecuredDebt

aci:TaxExemptBonds


Proceeds
Period End2021-06-302021-03-312020-09-30
Value9.28138
Percentage of ProceedsFromIssuanceOfSecuredDebt100.0100.0100.0

us-gaap:ProceedsFromRepaymentsOfOtherDebt

us-gaap:ProceedsFromRepaymentsOfOtherDebt


Proceeds
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312010-09-302010-06-302010-03-31
Value-9259000.0-9536000.0-5755000.0-8048000.0-5544000.0-3232000.0-4212000.0-4633000.04209000.0-2979000.0-3876000.0-3431000.05298000.0-785000.0-2397000.0-2810000.03374000.0-1790000.0-4883000.0-5410000.0860000.0-2366000.0-5016000.0-4810000.04792000.0-797000.0-4869000.0-4521000.06376000.0-1252000.0-5484000.0-5964000.012396000.0-1914000.0-5171000.0-5993000.014126000.053000.0-3684000.0-5161000.09876000.0-1545000.0-3507000.0-4742000.0
Percentage of ProceedsFromRepaymentsOfOtherDebt100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:ProceedsFromRepaymentsOfOtherLongTermDebt

us-gaap:ProceedsFromRepaymentsOfOtherLongTermDebt


Proceeds
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312013-12-312013-09-302012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302010-12-31
Value-0.75-0.75-0.75-0.75-0.75-0.75-0.75-0.75-0.75-0.75-0.75-0.75-0.750.0-628.788-0.129-0.128-0.152-451.324-1.331209-296.1121011
Percentage of ProceedsFromRepaymentsOfOtherLongTermDebt100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities

us-gaap:ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities


Proceeds
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302016-12-31
Value343410626223865563263.23285549663823
Percentage of ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:ProfessionalFees us-gaap:ScheduleOfEquityMethodInvestmentEquityMethodInvesteeName

us-gaap:ProfessionalFees

aci:JointVenturePowderRiverBasinAndColoradoMiningOperations


Professional
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-30
Value0.200000000000001874.47.93.77.03.8
Percentage of ProfessionalFees100.0100.0100.0100.0100.0100.0

us-gaap:ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation, us-gaap:Position

us-gaap:ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax

us-gaap:CoalContract, us-gaap:DesignatedAsHedgingInstrument, us-gaap:CostOfSales, us-gaap:Long


Reclassification
Period End2020-09-302020-06-302020-03-312019-09-302019-06-302019-03-31
Value471000.0706000.0000-683000.0
Percentage of ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax100.0100.00.00.00.0100.0

us-gaap:CoalContract, us-gaap:DesignatedAsHedgingInstrument, us-gaap:SalesRevenueNet, us-gaap:Short


Reclassification
Period End2020-09-302020-06-302019-09-302019-06-302019-03-31
Value-569000.0-902000.02963000.01743000.01044000.0
Percentage of ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax-120.8-127.80.00.0-152.9

us-gaap:ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax us-gaap:HedgingDesignation

us-gaap:DesignatedAsHedgingInstrument


Reclassification
Period End2020-09-302020-06-302019-09-302019-06-302019-03-31
Value-98000.0-196000.02963000.01743000.0361000.0
Percentage of ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax-20.8-27.80.00.0-52.9

us-gaap:ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax

us-gaap:ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax


Reclassification
Period End2021-03-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value-1.161-0.0850.0820.045.9025.5922.7010.876-2.621-3.9381.7350.141.169-0.856-0.8890.823-1.359-1.6726.7821.2540.8291.020.499
Percentage of ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax us-gaap:StatementEquityComponents

us-gaap:AccumulatedDefinedBenefitPlansAdjustment


Reclassification
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value0.441-0.5470.220.3290.3930.2571.4842.4330.4290.00.3480.6131.3710.01.176-0.9050.4870.0-2.2432.7490.857-1.974-0.757-2.222-0.1892.3270.7751.4521.1820.336
Percentage of ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax0.047.10.0-387.1479.3642.525.143.515.90.0-13.3-15.679.00.0100.60.00.00.00.00.0-100.1222.0-92.0163.511.334.361.8175.2115.967.3

us-gaap:AccumulatedGainLossNetCashFlowHedgeParent


Reclassification
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-31
Value-0.632-0.624-0.54-0.541-0.452-0.216
Percentage of ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax0.053.70.0636.5-551.2-540.0

us-gaap:AccumulatedNetUnrealizedInvestmentGainLoss


Reclassification
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value0.0140.01-0.0030.1280.141-0.0010.1350.0350.0150.00.00.0080.0160.00.0550.00.00.3320.6320.0-1.857-0.689-0.723-0.881-1.793.624-0.092-0.719-0.356-0.345
Percentage of ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax0.0-0.90.0-150.6172.0-2.52.30.60.60.0-0.0-0.20.90.04.70.00.00.00.00.0216.977.5-87.864.8107.153.4-7.3-86.7-34.9-69.1

us-gaap:SeveranceCosts1

us-gaap:SeveranceCosts1


Severance
Period End2020-12-312020-09-302020-06-302020-03-312016-06-302016-03-31
Value0.090.0187.4375.8282.15.1
Percentage of SeveranceCosts1100.0100.0100.0100.0100.0100.0

us-gaap:StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures

us-gaap:StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures


Stock
Period End2021-03-31
Value59166.0
Percentage of StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures100.0

us-gaap:StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures

us-gaap:StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures


Stock
Period End2021-03-31
Value1000.0
Percentage of StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures100.0

us-gaap:StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures us-gaap:StatementEquityComponents

us-gaap:CommonStock


Stock
Period End2021-03-31
Value1000.0
Percentage of StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures100.0

us-gaap:StockRepurchasedAndRetiredDuringPeriodValue

us-gaap:StockRepurchasedAndRetiredDuringPeriodValue


Stock
Period End2021-03-312020-09-302020-06-302020-03-31
Value1317000.015000.050000.0198000.0
Percentage of StockRepurchasedAndRetiredDuringPeriodValue100.0100.0100.0100.0

us-gaap:StockRepurchasedAndRetiredDuringPeriodValue us-gaap:StatementEquityComponents

us-gaap:AdditionalPaidInCapital


Stock
Period End2021-03-312020-09-302020-06-302020-03-31
Value1317000.015000.050000.0198000.0
Percentage of StockRepurchasedAndRetiredDuringPeriodValue100.0100.0100.0100.0

us-gaap:UnrealizedGainLossOnDerivatives us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation, us-gaap:IncomeStatementLocation

us-gaap:UnrealizedGainLossOnDerivatives

us-gaap:CoalContract, us-gaap:Nondesignated, aci:ChangeInFairValueOfCoalDerivativesAndCoalTradingActivityNetDomain


Unrealized
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value-8.762-0.528-1.956-2.650.129-1.04-1.432-1.4179.13713
Percentage of UnrealizedGainLossOnDerivatives100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:WeightedAverageNumberDilutedSharesOutstandingAdjustment

us-gaap:WeightedAverageNumberDilutedSharesOutstandingAdjustment


Weighted
Period End2021-06-302019-09-302019-06-302019-03-312018-09-302018-06-302018-03-312017-09-302017-06-302017-03-312016-12-31
Value1462000.01116000.01238000.01105000.0958000.0880000.0974000.0555000.0423000.0400000.0467000.0
Percentage of WeightedAverageNumberDilutedSharesOutstandingAdjustment100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:CoalLandsAndMineralRights

aci:CoalLandsAndMineralRights


Coal
Period End2020-12-312019-12-312011-12-312010-12-312009-12-31
Value406450657825232417
Percentage of CoalLandsAndMineralRights100.0100.0100.0100.0100.0

aci:DeferredMineDevelopment

aci:DeferredMineDevelopment


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312011-12-312010-12-312009-12-31
Value2882521942672805538911064872832
Percentage of DeferredMineDevelopment100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTaxAndOtherTaxCredits

aci:DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTaxAndOtherTaxCredits


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value3.1175.2975.3942922
Percentage of DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTaxAndOtherTaxCredits100.0100.0100.0100.0100.0

aci:DefinedBenefitPlanExpectedFutureBenefitPaymentsTotal us-gaap:RetirementPlanType

aci:DefinedBenefitPlanExpectedFutureBenefitPaymentsTotal

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value121118118127
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsTotal100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value117137160174
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsTotal96.3116.0135.9137.0

aci:DefinedBenefitPlanFairValueOfPlanAssetsNet

aci:DefinedBenefitPlanFairValueOfPlanAssetsNet


Defined
Period End2020-12-312019-12-31
Value199211
Percentage of DefinedBenefitPlanFairValueOfPlanAssetsNet100.0100.0

aci:DerivativeAssetExcludingInterestRateSwaps

aci:DerivativeAssetExcludingInterestRateSwaps


Derivative
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302016-12-31
Value0.4710.8532.6655.6595.1595.8579.983155.3671.0787.6757.8677.5635.5082.0314.908
Percentage of DerivativeAssetExcludingInterestRateSwaps100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:IncomeTaxRefundReserve

aci:IncomeTaxRefundReserve


Income
Period End2020-09-30
Value13
Percentage of IncomeTaxRefundReserve100.0

aci:LeaseAssets

aci:LeaseAssets


Lease
Period End2021-06-302021-03-312020-12-31
Value202122
Percentage of LeaseAssets100.0100.0100.0

aci:LeaseLiabilities

aci:LeaseLiabilities


Lease
Period End2021-06-302021-03-312020-12-31
Value212223
Percentage of LeaseLiabilities100.0100.0100.0

aci:LineOfCreditFacilityCovenantComplianceLiquidityAmount us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType

aci:LineOfCreditFacilityCovenantComplianceLiquidityAmount

us-gaap:LineOfCredit, aci:InventoryBasedRevolvingCreditFacility, aci:RegionsBank, us-gaap:LineOfCredit


Line
Period End2020-09-302020-08-31
Value100175
Percentage of LineOfCreditFacilityCovenantComplianceLiquidityAmount100.0100.0

aci:OperatingLossCarryforwardsAndTaxCreditCarryforwardsLimitationsOnUseAnnualAmount

aci:OperatingLossCarryforwardsAndTaxCreditCarryforwardsLimitationsOnUseAnnualAmount


Operating
Period End2020-12-31
Value29
Percentage of OperatingLossCarryforwardsAndTaxCreditCarryforwardsLimitationsOnUseAnnualAmount100.0

aci:OtherAssetsNonCurrentTotal

aci:OtherAssetsNonCurrentTotal


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-31
Value137132127125171182176
Percentage of OtherAssetsNonCurrentTotal100.0100.0100.0100.0100.0100.0100.0

aci:PercentageOfAccountsReceivableNetCurrent us-gaap:ConcentrationRiskByType

aci:PercentageOfAccountsReceivableNetCurrent

aci:DomesticAndForeignSteelProducers


Percentage
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-31
Value62.00.590.630.580.480.280.36
Percentage of PercentageOfAccountsReceivableNetCurrent100.0100.0100.0100.0100.0100.0100.0

aci:RevenueRemainingPerformanceObligationFixedPriceVolume

aci:RevenueRemainingPerformanceObligationFixedPriceVolume


Revenue
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-31
Value3136163648173859187051
Percentage of RevenueRemainingPerformanceObligationFixedPriceVolume100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:RevenueRemainingPerformanceObligationFixedPriceVolume us-gaap:ProductOrService

us-gaap:PublicUtilitiesInventoryCoal


Revenue
Period End2020-12-312019-12-31
Value4961
Percentage of RevenueRemainingPerformanceObligationFixedPriceVolume0.00.0

aci:RevenueRemainingPerformanceObligationMass us-gaap:ProductOrService

aci:RevenueRemainingPerformanceObligationMass

us-gaap:PublicUtilitiesInventoryCoal


Revenue
Period End2020-12-312019-12-31
Value5667
Percentage of RevenueRemainingPerformanceObligationMass100.0100.0

aci:RevenueRemainingPerformanceObligationVariablePriceVolume

aci:RevenueRemainingPerformanceObligationVariablePriceVolume


Revenue
Period End2021-06-302021-03-312020-09-302020-06-302020-03-312019-09-302019-06-302019-03-312018-09-302018-06-302018-03-31
Value4400000.06700000.01000000.02500000.01800000.01800000.04900000.04000000.03000000.06400000.04000000.0
Percentage of RevenueRemainingPerformanceObligationVariablePriceVolume100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

aci:RevenueRemainingPerformanceObligationVariablePriceVolume us-gaap:ProductOrService

us-gaap:PublicUtilitiesInventoryCoal


Revenue
Period End2020-12-312019-12-31
Value7100000.05800000.0
Percentage of RevenueRemainingPerformanceObligationVariablePriceVolume0.00.0

aci:SuretyBondsOutstanding

aci:SuretyBondsOutstanding


Surety
Period End2020-12-312019-12-312018-12-312017-12-31
Value31293031
Percentage of SuretyBondsOutstanding100.0100.0100.0100.0

aci:SuretyBondsOutstanding us-gaap:RegulatoryLiability

aci:AssetRetirementObligations


Surety
Period End2020-12-312019-12-312018-12-312017-12-312015-12-31
Value552528536531155
Percentage of SuretyBondsOutstanding1742.61792.91763.81704.20.0

us-gaap:AccrualForTaxesOtherThanIncomeTaxesCurrent

us-gaap:AccrualForTaxesOtherThanIncomeTaxesCurrent


Accrual
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302011-12-312010-12-31
Value525356606063697172707572787677747888921009797104111110111114114116112114113119121121123133107
Percentage of AccrualForTaxesOtherThanIncomeTaxesCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AllowanceForDoubtfulAccountsReceivableCurrent

us-gaap:AllowanceForDoubtfulAccountsReceivableCurrent


Allowance
Period End2021-06-302021-03-312020-12-312020-09-302016-09-302016-06-302016-03-312015-12-312014-12-31
Value0001303000.007800000.07800000.07800000.0200000.0
Percentage of AllowanceForDoubtfulAccountsReceivableCurrent0.00.00.0100.00.0100.0100.0100.0100.0

us-gaap:AssetRetirementObligation

us-gaap:AssetRetirementObligation


Asset
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-312009-12-31
Value257252243328356410418427448473343310
Percentage of AssetRetirementObligation100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AssetRetirementObligationCurrent

us-gaap:AssetRetirementObligationCurrent


Asset
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302011-12-312010-12-31
Value2727279.8279.82710101212121319191919161719191717131319191919222222243838383844278.862
Percentage of AssetRetirementObligationCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AssetRetirementObligationsNoncurrent

us-gaap:AssetRetirementObligationsNoncurrent


Asset
Period End2020-12-312019-12-312018-12-312017-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-31
Value230242230308396398402409446334
Percentage of AssetRetirementObligationsNoncurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AssetsFairValueDisclosure

us-gaap:AssetsFairValueDisclosure


Assets
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-03-312018-12-312018-09-302018-06-302017-12-312015-12-312014-12-312014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-30
Value33679765721341411701661731721632102672812782842882952832711616845311937
Percentage of AssetsFairValueDisclosure100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AssetsFairValueDisclosure us-gaap:FairValueByFairValueHierarchyLevel

us-gaap:FairValueInputsLevel1


Assets
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-03-312018-12-312018-09-302018-06-302017-12-312015-12-312014-12-312014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-30
Value1138571719434097999955641714262592124124119125985928196.36912
Percentage of AssetsFairValueDisclosure33.356.258.627.226.332.328.557.160.057.332.139.38.25.39.59.332.643.042.342.246.361.487.661.562.832.733.7

us-gaap:FairValueInputsLevel2


Assets
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-03-312018-12-312018-09-302018-06-302017-12-312015-12-312014-12-312014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-30
Value22294047539010172656711093191250248248186160166158136481.652.0681.452.69510
Percentage of AssetsFairValueDisclosure66.743.841.472.873.667.671.542.439.639.164.157.490.693.688.489.365.655.556.455.750.330.12.44.54.613.827.0

us-gaap:AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax

us-gaap:AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax


Available
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value17000.012000.014000.0190000.0245000.0201000.0327000.0341000.0156000.015000.01000.06000.022000.0
Percentage of AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax us-gaap:FinancialInstrument

us-gaap:CorporateDebtSecurities


Available
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value1000.014000.0166000.0159000.0200000.0275000.0247000.098000.04000.003000.00
Percentage of AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax8.3100.087.464.999.584.172.462.826.70.050.00.0

us-gaap:USGovernmentCorporationsAndAgenciesSecurities


Available
Period End2020-12-312019-12-312018-12-312017-12-31
Value11000.01000.011000.022000.0
Percentage of AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax91.70.573.3100.0

us-gaap:USTreasuryAndGovernment


Available
Period End2021-03-312020-12-312020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value17000.011000.024000.086000.01000.052000.094000.058000.011000.01000.03000.022000.0
Percentage of AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax100.091.712.635.10.515.927.637.273.3100.050.0100.0

us-gaap:AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax

us-gaap:AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value267000.0267000.0363000.0223000.0144000.0732000.0221000.0118000.034000.0122000.0358000.0380000.0486000.0365000.0
Percentage of AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax us-gaap:FinancialInstrument

us-gaap:CorporateDebtSecurities


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value176000.0200000.0277000.0169000.0144000.0732000.0205000.0101000.034000.085000.0232000.0200000.0325000.0292000.0
Percentage of AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax65.974.976.375.8100.0100.092.885.6100.069.764.852.666.980.0

us-gaap:USGovernmentCorporationsAndAgenciesSecurities


Available
Period End2020-12-312019-12-312018-12-312017-12-31
Value86000.016000.0126000.073000.0
Percentage of AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax23.77.235.220.0

us-gaap:USTreasuryAndGovernment


Available
Period End2021-06-302021-03-312020-12-312020-09-302019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value91000.067000.086000.054000.016000.017000.0037000.0126000.0180000.0161000.073000.0
Percentage of AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax34.125.123.724.27.214.40.030.335.247.433.120.0

us-gaap:AvailableForSaleDebtSecuritiesAmortizedCostBasis

us-gaap:AvailableForSaleDebtSecuritiesAmortizedCostBasis


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value3367976367129135175162164163162161156
Percentage of AvailableForSaleDebtSecuritiesAmortizedCostBasis100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AvailableForSaleDebtSecuritiesAmortizedCostBasis us-gaap:FinancialInstrument

us-gaap:CorporateDebtSecurities


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value2229394753911001101047063637292
Percentage of AvailableForSaleDebtSecuritiesAmortizedCostBasis66.743.641.075.779.170.474.263.164.142.538.738.844.958.9

us-gaap:USGovernmentCorporationsAndAgenciesSecurities


Available
Period End2020-12-312019-12-312018-12-312017-12-31
Value573510064
Percentage of AvailableForSaleDebtSecuritiesAmortizedCostBasis59.025.861.341.1

us-gaap:USTreasuryAndGovernment


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value11385715143835645894100998864
Percentage of AvailableForSaleDebtSecuritiesAmortizedCostBasis33.356.459.024.320.929.625.836.935.957.561.361.255.141.1

us-gaap:AvailableForSaleSecuritiesDebtSecurities

us-gaap:AvailableForSaleSecuritiesDebtSecurities


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value3367966367129135176162164162162160155
Percentage of AvailableForSaleSecuritiesDebtSecurities100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:AvailableForSaleSecuritiesDebtSecurities us-gaap:FairValueByFairValueHierarchyLevel

us-gaap:FairValueInputsLevel1


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-03-312018-12-312018-09-302018-06-302017-12-31
Value113857151438359499995464
Percentage of AvailableForSaleSecuritiesDebtSecurities33.356.659.124.220.929.725.857.561.461.334.141.1

us-gaap:FairValueInputsLevel2


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-03-312018-12-312018-09-302018-06-302017-12-31
Value22293947539010070626210691
Percentage of AvailableForSaleSecuritiesDebtSecurities66.743.440.975.879.170.374.242.538.638.765.958.9

us-gaap:AvailableForSaleSecuritiesDebtSecurities us-gaap:FinancialInstrument

us-gaap:CorporateDebtSecurities


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value2229394753901001111047062627291
Percentage of AvailableForSaleSecuritiesDebtSecurities66.743.440.975.879.170.374.263.164.142.538.638.744.958.9

us-gaap:USGovernmentCorporationsAndAgenciesSecurities


Available
Period End2020-12-312019-12-312018-12-312017-12-31
Value57359964
Percentage of AvailableForSaleSecuritiesDebtSecurities59.125.861.441.1

us-gaap:USTreasuryAndGovernment


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value1138571514383564589499998864
Percentage of AvailableForSaleSecuritiesDebtSecurities33.356.659.124.220.929.725.836.935.957.561.461.355.141.1

us-gaap:AvailableForSaleSecuritiesDebtSecuritiesCurrent

us-gaap:AvailableForSaleSecuritiesDebtSecuritiesCurrent


Available
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-30
Value33679663671291351761621641621621601441551541591228811120319820019924924924824924824824824824824823499
Percentage of AvailableForSaleSecuritiesDebtSecuritiesCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:CashCollateralForBorrowedSecurities us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LongtermDebtType

us-gaap:CashCollateralForBorrowedSecurities

us-gaap:LineOfCredit, aci:AccountsReceivableSecuritizationFacility, us-gaap:SecuredDebt


Cash
Period End2021-03-31
Value2600000.0
Percentage of CashCollateralForBorrowedSecurities100.0

us-gaap:ConvertibleDebtCurrent us-gaap:DebtInstrument

us-gaap:ConvertibleDebtCurrent

aci:ConvertibleSeniorNotesDue2025


Convertible
Period End2021-06-30
Value114
Percentage of ConvertibleDebtCurrent100.0

us-gaap:DebtInstrumentCarryingAmount

us-gaap:DebtInstrumentCarryingAmount


Debt
Period End2020-12-31
Value559
Percentage of DebtInstrumentCarryingAmount100.0

us-gaap:DebtInstrumentCarryingAmount us-gaap:DebtInstrument

aci:OtherDebt


Debt
Period End2021-06-302021-03-312020-12-312020-09-302019-12-31
Value4353625925
Percentage of DebtInstrumentCarryingAmount0.00.011.20.00.0

aci:TaxExemptBonds


Debt
Period End2021-06-302021-03-312020-12-312020-09-30
Value98985353
Percentage of DebtInstrumentCarryingAmount0.00.09.50.0

aci:TermLoanDue2024


Debt
Period End2021-06-302021-03-312020-12-312020-09-302019-12-31
Value286287288288290
Percentage of DebtInstrumentCarryingAmount0.00.051.50.00.0

us-gaap:ConvertibleDebt


Debt
Period End2021-06-302021-03-312020-12-31
Value118116115
Percentage of DebtInstrumentCarryingAmount0.00.020.6

us-gaap:DebtInstrumentConvertibleConversionPrice1 us-gaap:DebtInstrument

us-gaap:DebtInstrumentConvertibleConversionPrice1

aci:ConvertibleSeniorNotesDue2025


Debt
Period End2020-11-03
Value37.325
Percentage of DebtInstrumentConvertibleConversionPrice1100.0

us-gaap:DebtInstrumentFaceAmount us-gaap:DebtInstrument

us-gaap:DebtInstrumentFaceAmount

aci:ConvertibleSeniorNotesDue2025


Debt
Period End2020-11-03
Value155
Percentage of DebtInstrumentFaceAmount100.0

aci:TaxExemptBonds


Debt
Period End2021-06-302021-03-312020-12-312020-07-02
Value98985353
Percentage of DebtInstrumentFaceAmount0.00.00.00.0

aci:TaxExemptBonds2020


Debt
Period End2020-07-02
Value53
Percentage of DebtInstrumentFaceAmount0.0

aci:TaxExemptBonds2021


Debt
Period End2021-03-04
Value45
Percentage of DebtInstrumentFaceAmount0.0

aci:TermLoanDue2024


Debt
Period End2021-06-302021-03-312020-12-312020-09-30
Value287288288289
Percentage of DebtInstrumentFaceAmount0.00.00.00.0

us-gaap:DebtInstrumentFaceAmount us-gaap:DebtInstrument, us-gaap:SubsequentEventType

us-gaap:Bonds, us-gaap:SubsequentEvent


Debt
Period End2020-07-02
Value53
Percentage of DebtInstrumentFaceAmount0.0

us-gaap:DebtInstrumentFairValue

us-gaap:DebtInstrumentFairValue


Debt
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-03-312011-12-312011-09-302011-06-302010-12-31
Value648576533356308300308306314319318322327332336332331334362150092871293714001900240027003600450045004600450047005000500039004200390043001700
Percentage of DebtInstrumentFairValue100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DebtInstrumentMeasurementInput aci:DefinedBenefitPlansBenefitObligationExpenses, us-gaap:MeasurementInputType

us-gaap:DebtInstrumentMeasurementInput

aci:WorkersCompensation, us-gaap:MeasurementInputRiskFreeInterestRate


Debt
Period End2020-12-312019-12-312018-12-31
Value0.840.01750.0308
Percentage of DebtInstrumentMeasurementInput100.0100.0100.0

us-gaap:DebtInstrumentMeasurementInput us-gaap:MeasurementInputType

us-gaap:MeasurementInputDiscountRate


Debt
Period End2020-12-312019-12-312018-12-312017-12-31
Value2.480.03310.04260.0366
Percentage of DebtInstrumentMeasurementInput295.2189.1138.30.0

us-gaap:DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger

us-gaap:DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger


Debt
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-06-302017-12-31
Value0.03.08.19.65.10.00.00.00.09.032170.0
Percentage of DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger0.0100.0100.0100.0100.00.00.00.00.0100.0100.0100.00.0

us-gaap:DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months

us-gaap:DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months


Debt
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302017-12-31
Value32404548125572512531115151128132
Percentage of DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DeferredFinanceCostsCurrentNet

us-gaap:DeferredFinanceCostsCurrentNet


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-31
Value0.72.02.22.31.96525191715
Percentage of DeferredFinanceCostsCurrentNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DeferredFinanceCostsGross

us-gaap:DeferredFinanceCostsGross


Deferred
Period End2021-06-302021-03-312020-12-312019-12-312018-12-312017-12-312011-12-312010-12-31
Value1111105.0136.0927.0329037
Percentage of DeferredFinanceCostsGross100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DeferredFinanceCostsNet

us-gaap:DeferredFinanceCostsNet


Deferred
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312014-12-312013-12-312012-12-312012-09-302012-06-302011-12-31
Value4.44.41.96.9144.9564.7383.05.2895.5625.834.76.3576.6196.7825.37.2866.827.0085.1826464-64.8571128999101272790
Percentage of DeferredFinanceCostsNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DeferredFinanceCostsNet us-gaap:DebtInstrument

aci:ConvertibleSeniorNotesDue2025


Deferred
Period End2020-12-31
Value4.4
Percentage of DeferredFinanceCostsNet231.6

us-gaap:DeferredIncomeTaxLiabilities

us-gaap:DeferredIncomeTaxLiabilities


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-31
Value144.2414.1915.0252143414621477
Percentage of DeferredIncomeTaxLiabilities100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DeferredOfferingCosts us-gaap:DebtInstrument

us-gaap:DeferredOfferingCosts

aci:ConvertibleSeniorNotesDue2025


Deferred
Period End2020-11-03
Value5100000.0
Percentage of DeferredOfferingCosts100.0

us-gaap:DeferredTaxAssetsGross

us-gaap:DeferredTaxAssetsGross


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-31
Value58851053463810431570136211561011786513
Percentage of DeferredTaxAssetsGross100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DeferredTaxAssetsInvestmentInSubsidiaries

us-gaap:DeferredTaxAssetsInvestmentInSubsidiaries


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value213190193308604
Percentage of DeferredTaxAssetsInvestmentInSubsidiaries100.0100.0100.0100.0100.0

us-gaap:DeferredTaxAssetsOperatingLossCarryforwards

us-gaap:DeferredTaxAssetsOperatingLossCarryforwards


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-31
Value35229430727110221086871126150324102
Percentage of DeferredTaxAssetsOperatingLossCarryforwards100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration

us-gaap:DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration


Deferred
Period End2020-12-31
Value1000
Percentage of DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration100.0

us-gaap:DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAssetRetirementObligations

us-gaap:DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAssetRetirementObligations


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-31
Value19202828391211141131049371
Percentage of DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAssetRetirementObligations100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DeferredTaxLiabilitiesFinancingArrangements

us-gaap:DeferredTaxLiabilitiesFinancingArrangements


Deferred
Period End2020-12-31
Value8845000.0
Percentage of DeferredTaxLiabilitiesFinancingArrangements100.0

us-gaap:DeferredTaxLiabilitiesOtherFiniteLivedAssets

us-gaap:DeferredTaxLiabilitiesOtherFiniteLivedAssets


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-31
Value4.2182.1781.1541.35114419591-77.0136776
Percentage of DeferredTaxLiabilitiesOtherFiniteLivedAssets100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DeferredTaxLiabilitiesPropertyPlantAndEquipment

us-gaap:DeferredTaxLiabilitiesPropertyPlantAndEquipment


Deferred
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-31
Value1.2192.0633.0373.6747.332389135413641411
Percentage of DeferredTaxLiabilitiesPropertyPlantAndEquipment100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DefinedBenefitPensionPlanLiabilitiesNoncurrent

us-gaap:DefinedBenefitPensionPlanLiabilitiesNoncurrent


Defined
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value2.2742.0882.8796.10813185.476121115165.1017.48113141732373823192027121314161513107.11168666967454949481636384968
Percentage of DefinedBenefitPensionPlanLiabilitiesNoncurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DefinedBenefitPlanAccumulatedBenefitObligation us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanAccumulatedBenefitObligation

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value202217228270313
Percentage of DefinedBenefitPlanAccumulatedBenefitObligation100.0100.0100.0100.0100.0

us-gaap:DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value-2.226-20.333-25.936-5.137
Percentage of DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value-11.67-15.529-8.899-16.178
Percentage of DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeBeforeTax524.376.434.3314.9

us-gaap:DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value2.22620255.137
Percentage of DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value10148.89916
Percentage of DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetGainsLossesBeforeTax484.771.534.3314.9

us-gaap:DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-31
Value-0.88-0.9920.0
Percentage of DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax100.0100.00.0

us-gaap:DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate us-gaap:RetirementPlanName, us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate

aci:CashBalance, us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value2.190.03090.04110.0349
Percentage of DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate100.0100.0100.0100.0

aci:ExcessPlan, us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value1.960.0290.03940.0327
Percentage of DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate89.593.995.993.7

us-gaap:DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate us-gaap:RetirementPlanType

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value2.170.03094.123.49
Percentage of DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate99.1100.010024.310000.0

us-gaap:DefinedBenefitPlanBenefitObligation us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanBenefitObligation

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value1008788110111
Percentage of DefinedBenefitPlanBenefitObligation100.0100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value202217228270313
Percentage of DefinedBenefitPlanBenefitObligation200.5247.6258.4244.4280.4

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value60585762
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value52607383
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter86.7102.4127.0132.2

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value12121112
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value13181617
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths105.0149.6136.7142.3

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value12111213
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value13131619
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive110.1115.2140.0143.3

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value12111213
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value12141818
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour99.9118.9151.2140.8

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value12121212
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value12151818
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree102.1126.4150.7139.9

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value12121212
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value13161717
Percentage of DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo111.0135.3143.2142.1

us-gaap:DefinedBenefitPlanFundedStatusOfPlan us-gaap:RetirementPlanType

us-gaap:DefinedBenefitPlanFundedStatusOfPlan

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value-100900000.0-87867000.0-88563000.0-110520000.0
Percentage of DefinedBenefitPlanFundedStatusOfPlan100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Defined
Period End2020-12-312019-12-312018-12-312017-12-31
Value-3019000.0-5746000.0-16367000.0-14456000.0
Percentage of DefinedBenefitPlanFundedStatusOfPlan3.06.518.513.1

us-gaap:DefinedBenefitPlanHealthCareCostTrendRateAssumedNextFiscalYear

us-gaap:DefinedBenefitPlanHealthCareCostTrendRateAssumedNextFiscalYear


Defined
Period End2020-12-31
Value5.7
Percentage of DefinedBenefitPlanHealthCareCostTrendRateAssumedNextFiscalYear100.0

us-gaap:DefinedBenefitPlanUltimateHealthCareCostTrendRate1

us-gaap:DefinedBenefitPlanUltimateHealthCareCostTrendRate1


Defined
Period End2020-12-31
Value4.5
Percentage of DefinedBenefitPlanUltimateHealthCareCostTrendRate1100.0

us-gaap:DepositsAssetsNoncurrent

us-gaap:DepositsAssetsNoncurrent


Deposits
Period End2020-12-31
Value600000.0
Percentage of DepositsAssetsNoncurrent100.0

us-gaap:DerivativeAssetCollateralObligationToReturnCashOffset

us-gaap:DerivativeAssetCollateralObligationToReturnCashOffset


Derivative
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-31
Value2.3922517506910798
Percentage of DerivativeAssetCollateralObligationToReturnCashOffset100.0100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeAssetFairValueGrossLiability

us-gaap:DerivativeAssetFairValueGrossLiability


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302013-12-31
Value9.4532.4192.3927.75720262532725717424225504341436964437210712089136986947
Percentage of DerivativeAssetFairValueGrossLiability100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeAssets

us-gaap:DerivativeAssets


Derivative
Period End2020-12-312019-12-312019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value0.8535.8576.2491.0781112115.5082.2680.9631.4184.9086.1538.687.1899.05214181813171815192734273254603724143028
Percentage of DerivativeAssets100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeAssetsLiabilitiesAtFairValueNet

us-gaap:DerivativeAssetsLiabilitiesAtFairValueNet


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value-8.840.0010.8462.6125.6595.1585.7469.354145.367-12.21-28.88-18.2026.777-1.709-0.265-0.7130.7254.8075.7988.2047.1899.052141717121718151926342630515328168.3252523
Percentage of DerivativeAssetsLiabilitiesAtFairValueNet100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeAssetsLiabilitiesAtFairValueNet us-gaap:BalanceSheetLocation

us-gaap:AccruedLiabilities


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302015-12-31
Value-8.84-0.47-0.007-0.053-0.001-0.111-0.629-0.650.0-13.288-36.555-26.069-0.786-7.217-2.296-1.427-0.693-0.101-0.3550.0
Percentage of DerivativeAssetsLiabilitiesAtFairValueNet100.0-47000.0-0.8-2.0-0.0-1.9-6.7-4.40.0108.8126.6143.2-11.6422.3866.4200.1-95.6-2.1-6.10.0

us-gaap:DerivativeAssetsLiabilitiesAtFairValueNet us-gaap:BalanceSheetLocation, us-gaap:DerivativeInstrumentRisk

us-gaap:AccruedLiabilities, us-gaap:PublicUtilitiesInventoryCoal


Derivative
Period End2020-12-312019-12-31
Value-0.007-0.111
Percentage of DerivativeAssetsLiabilitiesAtFairValueNet-0.8-1.9

us-gaap:OtherCurrentAssets, aci:HeatingOilAndForeignCurrency


Derivative
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-31
Value0.4710.8532.6655.6595.1595.857
Percentage of DerivativeAssetsLiabilitiesAtFairValueNet47100.0100.8102.0100.0100.0101.9

us-gaap:OtherCurrentAssets, srt:HeatingOil


Derivative
Period End2020-12-312019-12-312018-12-31
Value0.8530.1330.532
Percentage of DerivativeAssetsLiabilitiesAtFairValueNet100.82.3-4.4

us-gaap:DerivativeAveragePriceRiskOptionStrikePrice us-gaap:ConcentrationRiskByType, us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation

us-gaap:DerivativeAveragePriceRiskOptionStrikePrice

aci:DieselPurchases, aci:NYMEX, us-gaap:Nondesignated


Derivative
Period End2020-09-302020-06-302020-03-312019-09-302019-06-302019-03-31
Value1.051.031.031.921.911.91
Percentage of DerivativeAveragePriceRiskOptionStrikePrice100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeAveragePriceRiskOptionStrikePrice us-gaap:HedgingDesignation, us-gaap:TradingActivityByType

us-gaap:Nondesignated, srt:HeatingOil


Derivative
Period End2020-09-302020-06-302020-03-312019-09-302019-06-30
Value1.761.741.82.32.34
Percentage of DerivativeAveragePriceRiskOptionStrikePrice167.6168.9174.8119.8122.5

us-gaap:DerivativeCollateralObligationToReturnCash

us-gaap:DerivativeCollateralObligationToReturnCash


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302014-12-31
Value102.61.42.13.91.84.41.92.11324544119169.34.64.82.81.40.30.91.72.90.82.4
Percentage of DerivativeCollateralObligationToReturnCash100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeFairValueOfDerivativeAsset

us-gaap:DerivativeFairValueOfDerivativeAsset


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312015-12-312014-12-312014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value9.4532.893.2451026313142886218505032554542447411611284846782957254911056842264151
Percentage of DerivativeFairValueOfDerivativeAsset100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeFairValueOfDerivativeAsset us-gaap:DerivativeInstrumentRisk

us-gaap:InterestRateSwap


Derivative
Period End2020-09-302020-06-302020-03-31
Value-4.5-5.1-5.6
Percentage of DerivativeFairValueOfDerivativeAsset-43.2-19.6-17.9

us-gaap:DerivativeFairValueOfDerivativeAsset us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation

aci:HeatingOilDieselPurchases, us-gaap:Nondesignated


Derivative
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value0.2370.1050.5280.2010.1330.3830.9231.7340.5326.246.6524.9275.3542.0060.6611.4184.646
Percentage of DerivativeFairValueOfDerivativeAsset7.31.02.00.60.40.91.02.82.812.413.215.09.64.41.53.26.3

us-gaap:CoalContract, us-gaap:DesignatedAsHedgingInstrument


Derivative
Period End2020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value0.4531.0471.1321.3513.8427.7285.4612.3420.311.7634.7960.9420.0030.0170.0090.00.00.0040.0020.0041.6084.6026.8966.5351.681.771.1010.909
Percentage of DerivativeFairValueOfDerivativeAsset4.34.03.64.39.08.88.712.40.63.514.61.70.00.00.00.00.00.00.00.00.00.00.05.80.02.11.31.3

us-gaap:CoalContract, us-gaap:Nondesignated


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value4.0590.9981.0944.7361110111833225.6725.0576.0583.145.1391.6240.0670.1930.475
Percentage of DerivativeFairValueOfDerivativeAsset42.934.533.745.444.433.436.942.938.435.930.010.012.09.69.33.50.20.40.6

us-gaap:PublicUtilitiesInventoryCoal, us-gaap:Nondesignated


Derivative
Period End2020-12-312019-12-312018-12-312017-12-31
Value1.094115.6725.139
Percentage of DerivativeFairValueOfDerivativeAsset33.736.930.09.3

us-gaap:DerivativeFairValueOfDerivativeAsset us-gaap:HedgingDesignation

us-gaap:Nondesignated


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312015-12-312014-12-312014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value9.4532.893.2459.96924303038805716504827544542447411610582836679937051861006532142035
Percentage of DerivativeFairValueOfDerivativeAsset100.0100.0100.095.796.096.495.791.091.291.387.699.496.585.498.3100.0100.0100.0100.0100.094.297.998.798.796.497.696.794.095.195.195.276.155.248.569.7

us-gaap:DerivativeFairValueOfDerivativeAsset us-gaap:HedgingDesignation, us-gaap:TradingActivityByType

us-gaap:Nondesignated, aci:CoalHeldForTradingPurposes


Derivative
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value1.9141810446811096555963411727372615101834
Percentage of DerivativeFairValueOfDerivativeAsset59.058.454.779.493.095.386.381.772.166.857.231.729.835.539.436.740.345.267.4

us-gaap:Nondesignated, us-gaap:CoalContract


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value5.3941.8921.9145.128121918204533103835194442414368664676110121851269675727555
Percentage of DerivativeFairValueOfDerivativeAsset57.165.559.049.249.662.458.447.251.852.654.777.071.260.779.492.098.096.493.00.00.00.095.30.00.00.086.30.086.589.581.7

us-gaap:DerivativeFairValueOfDerivativeLiability

us-gaap:DerivativeFairValueOfDerivativeLiability


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312015-12-312014-12-312014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value182.8892.3997.812026253373573179682557464344691079965694756614624-40.156523925181627
Percentage of DerivativeFairValueOfDerivativeLiability100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeFairValueOfDerivativeLiability us-gaap:DerivativeInstrumentRisk

us-gaap:InterestRateSwap


Derivative
Period End2021-06-302021-03-312020-12-312019-12-31
Value2.63.23.93.0
Percentage of DerivativeFairValueOfDerivativeLiability14.2110.8162.611.6

us-gaap:DerivativeFairValueOfDerivativeLiability us-gaap:DerivativeInstrumentRisk, us-gaap:HedgingDesignation

us-gaap:CoalContract, us-gaap:DesignatedAsHedgingInstrument


Derivative
Period End2020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value0.3550.8510.8390.9620.6710.6370.2970.8057.9799.3880.8842.1460.00.00.00.0150.0140.050.00.020.00.1320.1072.4920.00.360.2150.026
Percentage of DerivativeFairValueOfDerivativeLiability4.54.23.23.72.00.90.52.610.013.73.43.70.00.00.00.00.00.00.00.00.00.00.02.50.00.50.30.1

us-gaap:CoalContract, us-gaap:Nondesignated


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-31
Value131.2360.8042.496.6595.6995.8561125231931225.0199.8921.9150.1460.2710.58
Percentage of DerivativeFairValueOfDerivativeLiability71.442.833.531.932.721.722.633.234.340.163.039.533.419.317.34.20.30.60.8

us-gaap:PublicUtilitiesInventoryCoal, us-gaap:Nondesignated


Derivative
Period End2020-12-312019-12-312018-12-312017-12-31
Value0.8045.856199.892
Percentage of DerivativeFairValueOfDerivativeLiability33.522.663.017.3

us-gaap:DerivativeFairValueOfDerivativeLiability us-gaap:HedgingDesignation

us-gaap:Nondesignated


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312015-12-312014-12-312014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value182.8892.3997.4551925243272573071592555464344691079665694756604524-38.477513925171424
Percentage of DerivativeFairValueOfDerivativeLiability100.0100.0100.095.595.896.896.398.099.199.597.490.086.396.696.3100.0100.0100.0100.0100.097.599.599.799.9100.099.199.5100.095.897.699.5100.093.288.391.4

us-gaap:DerivativeFairValueOfDerivativeLiability us-gaap:HedgingDesignation, us-gaap:TradingActivityByType

us-gaap:Nondesignated, aci:CoalHeldForTradingPurposes


Derivative
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value1.59518104568104934552574216-25.171383319109.78824
Percentage of DerivativeFairValueOfDerivativeLiability66.573.234.479.099.197.993.895.993.194.191.469.762.774.184.176.855.759.688.1

us-gaap:Nondesignated, us-gaap:CoalContract


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-31
Value5.2341.6531.5954.965121918204633104036204544434368644371104114821259366636745
Percentage of DerivativeFairValueOfDerivativeLiability28.657.266.563.663.275.073.262.963.959.034.450.452.977.379.095.899.599.499.10.00.00.097.90.00.00.093.80.096.897.695.9

us-gaap:DerivativeLiabilities

us-gaap:DerivativeLiabilities


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value8.840.470.0070.0530.0010.1110.6290.650.01336260.7867.2172.2961.4270.6930.1010.3550.4760.00.00.5360.8311.010.7660.0560.00.00.0120.4210.542-0.643-1.737-3.4987.099.17.8285.8245.7914.947
Percentage of DerivativeLiabilities100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.00.0100.0100.0100.0100.0100.00.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeLiabilityCollateralRightToReclaimCashOffset

us-gaap:DerivativeLiabilityCollateralRightToReclaimCashOffset


Derivative
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-31
Value2.3922517506910798
Percentage of DerivativeLiabilityCollateralRightToReclaimCashOffset100.0100.0100.0100.0100.0100.0100.0

us-gaap:DerivativeLiabilityCollateralRightToReclaimCashOffset us-gaap:BalanceSheetLocation

us-gaap:OtherCurrentLiabilities


Derivative
Period End2020-12-312019-12-312016-12-312015-12-312014-12-31
Value1.44.42.82.42.4
Percentage of DerivativeLiabilityCollateralRightToReclaimCashOffset58.517.14.02.22.4

us-gaap:DerivativeLiabilityFairValueGrossAsset

us-gaap:DerivativeLiabilityFairValueGrossAsset


Derivative
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302013-12-31
Value9.4532.4192.3927.75720262532725717424225504341436964437210712089136986947
Percentage of DerivativeLiabilityFairValueGrossAsset100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:EmployeeRelatedLiabilitiesCurrent

us-gaap:EmployeeRelatedLiabilitiesCurrent


Employee
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302011-12-312010-12-31
Value4837394446445049493557484133534751415858585258525049737262586768626372806551
Percentage of EmployeeRelatedLiabilitiesCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized us-gaap:AwardType

us-gaap:EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized

us-gaap:RestrictedStockUnitsRSU


Employee
Period End2020-12-31
Value15
Percentage of EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized100.0

us-gaap:EnergyRelatedInventoryCoal

us-gaap:EnergyRelatedInventoryCoal


Energy
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value797449566762467983574077726354486049375161858511999117718310198117134177182180202267298206180189115
Percentage of EnergyRelatedInventoryCoal100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:EquityMethodInvestmentOwnershipPercentage us-gaap:Ownership

us-gaap:EquityMethodInvestmentOwnershipPercentage

aci:KnightHawk


Equity
Period End2020-12-312018-12-31
Value49.50.49
Percentage of EquityMethodInvestmentOwnershipPercentage100.0100.0

us-gaap:EquityMethodInvestments

us-gaap:EquityMethodInvestments


Equity
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-312008-12-31
Value74747170106106105107106105104105104104106106104104961641632012012062272252352322292232212232322462422372352302252242072041778787
Percentage of EquityMethodInvestments100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:EquityMethodInvestments us-gaap:ScheduleOfEquityMethodInvestmentEquityMethodInvesteeName

aci:DominionTerminalAssociates


Equity
Period End2020-12-312019-12-312018-12-312017-12-312016-10-01
Value151514158.581
Percentage of EquityMethodInvestments21.914.614.015.00.0

aci:KnightHawk


Equity
Period End2020-12-312019-12-312018-12-312017-12-312016-10-012015-12-312014-12-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-312009-12-312008-12-31
Value56909090871511581521571531531491451431381351351311314948
Percentage of EquityMethodInvestments78.185.486.085.00.075.167.269.070.365.862.261.561.460.860.259.960.463.474.056.854.8

us-gaap:FinanceLeaseLiability

us-gaap:FinanceLeaseLiability


Finance
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-31
Value5451000.05664000.05874000.06081000.06283000.06465000.0
Percentage of FinanceLeaseLiability100.0100.0100.0100.0100.0100.0

us-gaap:FinanceLeaseLiabilityCurrent

us-gaap:FinanceLeaseLiabilityCurrent


Finance
Period End2021-06-302021-03-312020-12-312020-09-30
Value888000.0874000.0860000.0847000.0
Percentage of FinanceLeaseLiabilityCurrent100.0100.0100.0100.0

us-gaap:FinanceLeaseLiabilityNoncurrent

us-gaap:FinanceLeaseLiabilityNoncurrent


Finance
Period End2021-06-302021-03-312020-12-312020-09-30
Value4563000.04790000.05014000.05234000.0
Percentage of FinanceLeaseLiabilityNoncurrent100.0100.0100.0100.0

us-gaap:FinanceLeaseLiabilityPaymentsDue

us-gaap:FinanceLeaseLiabilityPaymentsDue


Finance
Period End2021-06-302021-03-312020-12-312020-09-30
Value6347000.06649000.06952000.07254000.0
Percentage of FinanceLeaseLiabilityPaymentsDue100.0100.0100.0100.0

us-gaap:FinanceLeaseLiabilityPaymentsDueYearFive

us-gaap:FinanceLeaseLiabilityPaymentsDueYearFive


Finance
Period End2020-12-312020-09-30
Value2112000.02111000.0
Percentage of FinanceLeaseLiabilityPaymentsDueYearFive100.0100.0

us-gaap:FinanceLeaseLiabilityPaymentsDueYearFour

us-gaap:FinanceLeaseLiabilityPaymentsDueYearFour


Finance
Period End2021-06-302021-03-312020-12-312020-09-30
Value2111000.02111000.01210000.01210000.0
Percentage of FinanceLeaseLiabilityPaymentsDueYearFour100.0100.0100.0100.0

us-gaap:FinanceLeaseLiabilityPaymentsRemainderOfFiscalYear

us-gaap:FinanceLeaseLiabilityPaymentsRemainderOfFiscalYear


Finance
Period End2021-06-302021-03-312020-09-30
Value606000.0908000.0303000.0
Percentage of FinanceLeaseLiabilityPaymentsRemainderOfFiscalYear100.0100.0100.0

us-gaap:FinanceLeaseLiabilityUndiscountedExcessAmount

us-gaap:FinanceLeaseLiabilityUndiscountedExcessAmount


Finance
Period End2021-06-302021-03-312020-12-312020-09-30
Value896000.0985000.01078000.01173000.0
Percentage of FinanceLeaseLiabilityUndiscountedExcessAmount100.0100.0100.0100.0

us-gaap:FinanceLeaseRightOfUseAsset

us-gaap:FinanceLeaseRightOfUseAsset


Finance
Period End2021-06-302021-03-312020-12-312020-09-30
Value4863000.05187000.05500000.05900000.0
Percentage of FinanceLeaseRightOfUseAsset100.0100.0100.0100.0

us-gaap:InsuranceSettlementsReceivableCurrent

us-gaap:InsuranceSettlementsReceivableCurrent


Insurance
Period End2020-12-312017-12-312016-12-31
Value500000.03300000.0800000.0
Percentage of InsuranceSettlementsReceivableCurrent100.0100.0100.0

us-gaap:InsuranceSettlementsReceivableNoncurrent

us-gaap:InsuranceSettlementsReceivableNoncurrent


Insurance
Period End2020-12-31
Value13
Percentage of InsuranceSettlementsReceivableNoncurrent100.0

us-gaap:InterestPayableCurrent

us-gaap:InterestPayableCurrent


Interest
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-12-152015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302011-12-312010-12-31
Value3.1254.1482.7950.7990.1210.1270.1330.1310.2210.2250.1560.2370.2040.2540.2460.1560.130.2028.03216216212711990773077307732791880408342705552
Percentage of InterestPayableCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet

us-gaap:InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet


Interest
Period End2020-09-30
Value100000.0
Percentage of InterestRateCashFlowHedgeGainLossToBeReclassifiedDuringNext12MonthsNet100.0

us-gaap:InterestRateDerivativeAssetsAtFairValue

us-gaap:InterestRateDerivativeAssetsAtFairValue


Interest
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312018-12-312017-12-31
Value471000.0853000.02665000.05659000.05158000.05857000.03554000.07339000.0
Percentage of InterestRateDerivativeAssetsAtFairValue100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:InterestRateDerivativeAssetsAtFairValue us-gaap:FairValueByFairValueHierarchyLevel

us-gaap:FairValueInputsLevel1


Interest
Period End2020-12-312020-09-302020-06-302020-03-312019-12-312018-12-312017-12-31
Value21000.02560000.05131000.04958000.05269000.000
Percentage of InterestRateDerivativeAssetsAtFairValue2.596.190.796.190.00.00.0

us-gaap:FairValueInputsLevel2


Interest
Period End2021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312018-12-312017-12-31
Value471000.0832000.0105000.0495000.0111000.0527000.03022000.01985000.0
Percentage of InterestRateDerivativeAssetsAtFairValue100.097.53.98.72.29.085.027.0

us-gaap:InterestRateDerivativeLiabilitiesAtFairValue

us-gaap:InterestRateDerivativeLiabilitiesAtFairValue


Interest
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312018-12-312017-12-31
Value113.6733.8994.5455.0665.6243.065137.217
Percentage of InterestRateDerivativeLiabilitiesAtFairValue100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:InterestRateDerivativeLiabilitiesAtFairValue us-gaap:FairValueByFairValueHierarchyLevel

us-gaap:FairValueInputsLevel1


Interest
Period End2021-06-302021-03-312020-12-312020-06-302020-03-312018-12-312017-12-31
Value7.4180.470.0070.00.0137.263
Percentage of InterestRateDerivativeLiabilitiesAtFairValue65.012.80.20.00.099.7100.6

us-gaap:FairValueInputsLevel2


Interest
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312018-12-312017-12-31
Value3.9873.2033.8924.5455.0665.6243.0650.0360.026
Percentage of InterestRateDerivativeLiabilitiesAtFairValue35.087.299.8100.0100.0100.0100.00.30.4

us-gaap:InventoryPartsAndComponentsNetOfReserves

us-gaap:InventoryPartsAndComponentsNetOfReserves


Inventory
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value82807686868784918988848684797473767676107110110111119124122118125127126137143163170172178178174163165160119
Percentage of InventoryPartsAndComponentsNetOfReserves100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:InventoryValuationReserves

us-gaap:InventoryValuationReserves


Inventory
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value0.70.60.62.52.42.32.21.71.41.20.61.00.90.90.30.20.10.10.05.16.56.46.06.76.66.86.68.78.78.68.48.813141311111313131312
Percentage of InventoryValuationReserves100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.00.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths

us-gaap:LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths


Lessee
Period End2021-06-302021-03-312020-12-312020-09-30
Value3317000.03317000.03367000.03367000.0
Percentage of LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths100.0100.0100.0100.0

us-gaap:LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear

us-gaap:LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear


Lessee
Period End2021-06-302021-03-312020-09-30
Value1681000.02507000.0823000.0
Percentage of LesseeOperatingLeaseLiabilityPaymentsRemainderOfFiscalYear100.0100.0100.0

us-gaap:LettersOfCreditOutstandingAmount

us-gaap:LettersOfCreditOutstandingAmount


Letters
Period End2020-12-312019-12-312018-12-312017-12-312015-12-31
Value200.00.07.421
Percentage of LettersOfCreditOutstandingAmount100.00.00.0100.0100.0

us-gaap:LettersOfCreditOutstandingAmount us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType

us-gaap:LineOfCredit, aci:InventoryBasedRevolvingCreditFacility, aci:RegionsBank, us-gaap:SecuredDebt


Letters
Period End2021-06-302021-03-312020-12-312019-09-302019-06-302019-03-312018-09-30
Value32292935353535
Percentage of LettersOfCreditOutstandingAmount0.00.0146.00.00.00.00.0

us-gaap:LettersOfCreditOutstandingAmount us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LongtermDebtType

us-gaap:LineOfCredit, aci:AccountsReceivableSecuritizationFacility, us-gaap:SecuredDebt


Letters
Period End2021-06-302021-03-312020-12-31
Value585856
Percentage of LettersOfCreditOutstandingAmount0.00.0281.5

us-gaap:LettersOfCreditOutstandingAmount us-gaap:CreditFacility, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType

us-gaap:LineOfCredit, aci:RegionsBank, us-gaap:SecuredDebt


Letters
Period End2020-09-302020-06-302020-03-312019-12-312018-12-312018-06-302018-03-312017-12-312017-09-302017-06-30
Value29323235353535292821
Percentage of LettersOfCreditOutstandingAmount0.00.00.00.00.00.00.0391.90.00.0

us-gaap:LettersOfCreditOutstandingAmount us-gaap:CreditFacility, us-gaap:LongtermDebtType

us-gaap:LineOfCredit, us-gaap:SecuredDebt


Letters
Period End2020-09-302020-06-302020-03-312019-12-312018-12-312018-06-302018-03-312017-12-312017-09-302017-04-27
Value5015151574750.08589130
Percentage of LettersOfCreditOutstandingAmount0.00.00.00.00.00.00.01148.60.00.0

us-gaap:LiabilityForFuturePolicyBenefitsDisabilityAndAccidentBenefits

us-gaap:LiabilityForFuturePolicyBenefitsDisabilityAndAccidentBenefits


Liability
Period End2020-12-31
Value14
Percentage of LiabilityForFuturePolicyBenefitsDisabilityAndAccidentBenefits100.0

us-gaap:LineOfCreditFacilityCurrentBorrowingCapacity us-gaap:CreditFacility, us-gaap:DebtInstrument, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType

us-gaap:LineOfCreditFacilityCurrentBorrowingCapacity

us-gaap:LineOfCredit, aci:AccountsReceivableSecuritizationFacility, aci:ArchReceivableCompanyLLC, us-gaap:SecuredDebt


Line
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312018-12-31
Value37552534686910853
Percentage of LineOfCreditFacilityCurrentBorrowingCapacity100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:LineOfCredit, aci:AccountsReceivableSecuritizationFacility, aci:RegionsBank, us-gaap:SecuredDebt


Line
Period End2020-09-302020-06-30
Value110160
Percentage of LineOfCreditFacilityCurrentBorrowingCapacity318.8234.6

us-gaap:LineOfCreditFacilityCurrentBorrowingCapacity us-gaap:CreditFacility, us-gaap:LineOfCreditFacility, us-gaap:LongtermDebtType

us-gaap:LineOfCredit, aci:RegionsBank, us-gaap:SecuredDebt


Line
Period End2020-09-302020-06-302018-12-312018-06-302018-03-312017-04-27
Value110160160160160160
Percentage of LineOfCreditFacilityCurrentBorrowingCapacity318.8234.6299.10.00.00.0

us-gaap:LitigationReserve

us-gaap:LitigationReserve


Litigation
Period End2020-12-312019-12-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302011-12-31
Value0.10.20.20.40.40.30.20.20.21.32.22.92.82.82.84.13.71722222323302132323232117
Percentage of LitigationReserve100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths


Long
Period End2020-12-31
Value33
Percentage of LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths100.0

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive


Long
Period End2020-12-31
Value208
Percentage of LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive100.0

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour


Long
Period End2020-12-31
Value283
Percentage of LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour100.0

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree


Long
Period End2020-12-31
Value17
Percentage of LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree100.0

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo

us-gaap:LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo


Long
Period End2020-12-31
Value16
Percentage of LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo100.0

us-gaap:MineReclamationAndClosingLiabilityNoncurrent

us-gaap:MineReclamationAndClosingLiabilityNoncurrent


Mine
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value213224230239238239242238236233230319316312308334342339337394390401396414409404398400395390402411414410409426424432446415412336334305
Percentage of MineReclamationAndClosingLiabilityNoncurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:MineralRights

us-gaap:MineralRights


Mineral
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-31
Value290360338361400240047004800
Percentage of MineralRights100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:NontradeReceivablesCurrent us-gaap:GainContingenciesByNature

us-gaap:NontradeReceivablesCurrent

aci:PreferenceRightsLeaseApplication


Nontrade
Period End2020-12-312020-09-30
Value1727
Percentage of NontradeReceivablesCurrent100.0100.0

us-gaap:OperatingLeaseWeightedAverageDiscountRatePercent

us-gaap:OperatingLeaseWeightedAverageDiscountRatePercent


Operating
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-31
Value5.55.55.55.50.0550.0555.55.50.0560.056
Percentage of OperatingLeaseWeightedAverageDiscountRatePercent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OperatingLossCarryforwards

us-gaap:OperatingLossCarryforwards


Operating
Period End2020-12-312018-12-312017-12-312016-12-312013-12-312012-12-312011-12-312008-12-31
Value1300130098093118001300779145
Percentage of OperatingLossCarryforwards100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherPostretirementDefinedBenefitPlanLiabilitiesNoncurrent

us-gaap:OtherPostretirementDefinedBenefitPlanLiabilitiesNoncurrent


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value959594858680807879818310510610410210410110110187878999333434323938373944464645414242428887363743
Percentage of OtherPostretirementDefinedBenefitPlanLiabilitiesNoncurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:OtherReceivables

us-gaap:OtherReceivables


Other
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value3.3363.7643.0532.882.96192124324448261211292321221917151518211415203235443124718184717066888391444440
Percentage of OtherReceivables100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities us-gaap:RetirementPlanType

us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Pension
Period End2020-12-312019-12-312018-12-312017-12-31
Value6.517.35.48.15
Percentage of PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Pension
Period End2020-12-312019-12-312018-12-312017-12-31
Value0.140.270.220.42
Percentage of PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities2.23.74.15.2

us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent us-gaap:RetirementPlanType

us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent

us-gaap:OtherPostretirementBenefitPlansDefinedBenefit


Pension
Period End2020-12-312019-12-312018-12-312017-12-31
Value1008788110
Percentage of PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent100.0100.0100.0100.0

us-gaap:PensionPlansDefinedBenefit


Pension
Period End2020-12-312019-12-312018-12-312017-12-31
Value3.0195.7461614
Percentage of PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesCurrentAndNoncurrent3.06.518.513.1

us-gaap:PropertyPlantAndEquipmentOther

us-gaap:PropertyPlantAndEquipmentOther


Property
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-312009-12-31
Value7346395104454182359293528823391322523972261
Percentage of PropertyPlantAndEquipmentOther100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:RestrictedCashCurrent

us-gaap:RestrictedCashCurrent


Restricted
Period End2021-06-302021-03-312020-12-312020-09-302018-12-31
Value7.104185.953130.215
Percentage of RestrictedCashCurrent100.0100.0100.0100.0100.0

us-gaap:RestrictedCashCurrent us-gaap:DebtInstrument

aci:TaxExemptBonds


Restricted
Period End2021-06-302021-03-312020-12-312020-09-30
Value7.1166.013
Percentage of RestrictedCashCurrent99.986.0100.899.9

us-gaap:SelfInsuranceReserveNoncurrent aci:DefinedBenefitPlansBenefitObligationExpenses

us-gaap:SelfInsuranceReserveNoncurrent

aci:OccupationalDisease


Self
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312011-12-312010-12-31
Value1831581181221115417
Percentage of SelfInsuranceReserveNoncurrent100.0100.0100.0100.0100.0100.0100.0

aci:WorkersCompensation


Self
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312011-12-312010-12-31
Value76737581882924
Percentage of SelfInsuranceReserveNoncurrent42.146.463.566.379.754.3140.9

us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber us-gaap:AwardType

us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber

aci:PerformanceBasedRestrictedStockUnitsRSUs


Share
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value286000.0195000.0415000.0311000.0225000.0
Percentage of ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber100.0100.0100.0100.0100.0

aci:TimeBasedRestrictedStockUnitsRSUs


Share
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value341000.0238000.0388000.0240000.0159000.0
Percentage of ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber119.2122.193.577.270.7

us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue us-gaap:AwardType

us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue

aci:PerformanceBasedRestrictedStockUnitsRSUs


Share
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value71.44106.4485.376.7567.34
Percentage of ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue100.0100.0100.0100.0100.0

aci:TimeBasedRestrictedStockUnitsRSUs


Share
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value64.3887.584.1179.8778.6
Percentage of ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue90.182.298.6104.1116.7

us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized us-gaap:PlanName

us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized

aci:TwoThousandSixteenOmnibusIncentivePlan


Share
Period End2020-12-31
Value3000000.0
Percentage of ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized100.0

us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant us-gaap:PlanName

us-gaap:ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant

aci:TwoThousandSixteenOmnibusIncentivePlan


Share
Period End2020-12-31
Value2100000.0
Percentage of ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant100.0

us-gaap:StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1

us-gaap:StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1


Stock
Period End2020-12-31
Value223
Percentage of StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1100.0

us-gaap:TreasuryStockCommonShares

us-gaap:TreasuryStockCommonShares


Treasury
Period End2020-12-312019-12-312018-12-312017-12-312016-12-31
Value10107.2163.9770.0
Percentage of TreasuryStockCommonShares100.0100.0100.0100.00.0

us-gaap:TreasuryStockShares

us-gaap:TreasuryStockShares


Treasury
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302013-12-312011-12-312011-09-302011-06-302011-03-312010-12-312009-12-31
Value101010101010109.9558.7858.0887.2166.2155.3444.3843.9770.1520.1520.1520.1520.1521.5171.5171.5171.5171371.51181.5121.5121.5121.5121.5121.512
Percentage of TreasuryStockShares100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued

us-gaap:UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued


Unrecognized
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-312011-12-312010-12-31
Value2300000.0800000.0300000.0600000.0500000.01700000.01500000.01300000.01000000.0800000.0600000.0
Percentage of UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary

us-gaap:UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary


Unrecorded
Period End2020-12-312015-12-312014-12-312012-12-31
Value10094265216
Percentage of UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary100.0100.0100.0100.0

us-gaap:ValuationAllowancesAndReservesBalance us-gaap:ValuationAllowancesAndReservesType

us-gaap:ValuationAllowancesAndReservesBalance

us-gaap:AllowanceForCreditLoss


Valuation
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312016-10-01
Value10100.00.00.00.0
Percentage of ValuationAllowancesAndReservesBalance100.0100.00.00.00.00.0

us-gaap:InventoryValuationReserve


Valuation
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312016-10-012015-12-312014-12-312013-12-312012-12-312011-12-312010-12-312009-12-312008-12-31
Value0.5742.2160.6480.2610.00.05.9916.6258.4461213121312
Percentage of ValuationAllowancesAndReservesBalance5.420.80.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:ValuationAllowanceOfDeferredTaxAssets


Valuation
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312016-10-012015-12-312014-12-312013-12-312012-12-312011-12-312010-12-312009-12-312008-12-31
Value57350653061010211033113127043342.8310.7371.120.395
Percentage of ValuationAllowancesAndReservesBalance5396.74760.40.00.00.00.00.00.00.00.00.00.00.00.0

us-gaap:WorkersCompensationLiabilityCurrent

us-gaap:WorkersCompensationLiabilityCurrent


Workers
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302011-12-312010-12-31
Value131215151514161818182019191918151516151313151617171516161716121214121010116.659
Percentage of WorkersCompensationLiabilityCurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:WorkersCompensationLiabilityCurrentAndNoncurrent

us-gaap:WorkersCompensationLiabilityCurrentAndNoncurrent


Workers
Period End2020-12-312019-12-312018-12-312017-12-312016-12-312015-12-312014-12-312013-12-312012-12-31
Value2592311942031991291119092
Percentage of WorkersCompensationLiabilityCurrentAndNoncurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0

us-gaap:WorkersCompensationLiabilityNoncurrent

us-gaap:WorkersCompensationLiabilityNoncurrent


Workers
Period End2021-06-302021-03-312020-12-312020-09-302020-06-302020-03-312019-12-312019-09-302019-06-302019-03-312018-12-312018-09-302018-06-302018-03-312017-12-312017-09-302017-06-302017-03-312016-12-312016-09-302016-06-302016-03-312015-12-312015-09-302015-06-302015-03-312014-12-312014-09-302014-06-302014-03-312013-12-312013-09-302013-06-302013-03-312012-12-312012-09-302012-06-302012-03-312011-12-312011-09-302011-06-302010-12-31
Value24924924421921921821517417316817418018518518419218618418411911811311295979894787475788279818185827471646535
Percentage of WorkersCompensationLiabilityNoncurrent100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0100.0

Line plots across dimensions of each concept


TTM Greenblatt ROC
_
(None,)
T T M  Greenblatt  R O C_
us-gaap:ComprehensiveIncomeNetOfTax
us-gaap:StatementEquityComponents
(None,)
Comprehensive Income Net Of Taxus-gaap: Statement Equity Components
us-gaap:NetIncomeLoss
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents
('us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome', None)
Net Income Lossus-gaap: Reclassification Out Of Accumulated Other Comprehensive Income, us-gaap: Statement Equity Components
us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax
us-gaap:StatementGeographical
(None,)
Revenue From Contract With Customer Excluding Assessed Taxus-gaap: Statement Geographical
us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments
('us-gaap:OperatingSegments', None)
Revenue From Contract With Customer Excluding Assessed Taxus-gaap: Consolidation Items, us-gaap: Statement Business Segments
us-gaap:Assets
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments
('us-gaap:OperatingSegments', None)
Assetsus-gaap: Consolidation Items, us-gaap: Statement Business Segments
us-gaap:Assets
us-gaap:ConsolidationItems
(None,)
Assetsus-gaap: Consolidation Items
us-gaap:StockholdersEquity
us-gaap:StatementEquityComponents
(None,)
Stockholders Equityus-gaap: Statement Equity Components
us-gaap:PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
us-gaap:RetirementPlanType
(None,)
Pension And Other Postretirement Defined Benefit Plans Liabilities Noncurrentus-gaap: Retirement Plan Type
us-gaap:StockholdersEquity
us-gaap:StatementEquityComponents
(None,)
Stockholders Equityus-gaap: Statement Equity Components
us-gaap:AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue
us-gaap:StatementEquityComponents
(None,)
Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Valueus-gaap: Statement Equity Components
us-gaap:DepreciationDepletionAndAmortization
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments
('us-gaap:OperatingSegments', None)
Depreciation Depletion And Amortizationus-gaap: Consolidation Items, us-gaap: Statement Business Segments
us-gaap:NetIncomeLoss
us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome, us-gaap:StatementEquityComponents
('us-gaap:ReclassificationOutOfAccumulatedOtherComprehensiveIncome', None)
Net Income Lossus-gaap: Reclassification Out Of Accumulated Other Comprehensive Income, us-gaap: Statement Equity Components
us-gaap:PaymentsToAcquirePropertyPlantAndEquipment
us-gaap:ConsolidationItems, us-gaap:StatementBusinessSegments
('us-gaap:OperatingSegments', None)
Payments To Acquire Property Plant And Equipmentus-gaap: Consolidation Items, us-gaap: Statement Business Segments