1.0.0.3 false Condensed Consolidated Statements of Cash Flows (USD $) In Millions false 1 $ false false Unit1 Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares xbrli 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 2 $ false false Unit1 Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares xbrli 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 7 Cash Flows From Operating Activities 5 us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperationsAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false No definition available. false 8 Net income 6 us-gaap_ProfitLoss us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 true true 368000000 368.0 false false 2 true true 665500000 665.5 false false No definition available. No authoritative reference available. false 9 (Income) loss from discontinued operations, net of income taxes 6 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -23600000 -23.6 false false 2 false true 42100000 42.1 false false No definition available. No authoritative reference available. true 10 Income from continuing operations, net of income taxes 6 us-gaap_IncomeLossFromContinuingOperationsIncludingPortionAttributableToNoncontrollingInterest us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 344400000 344.4 false false 2 false true 707600000 707.6 false false No definition available. No authoritative reference available. false 15 Adjustments to reconcile income from continuing operations, net of income taxes to net cash provided by operating activities: 10 us-gaap_AmortizationOfFinancingCostsAndDiscountsAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false No definition available. false 16 Depreciation, depletion and amortization 11 us-gaap_DepreciationDepletionAndAmortization us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 305500000 305.5 false false 2 false true 284400000 284.4 false false No definition available. No authoritative reference available. false 17 Deferred income taxes 11 us-gaap_DeferredIncomeTaxExpenseBenefit us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 99600000 99.6 false false 2 false true -25700000 -25.7 false false No definition available. No authoritative reference available. false 18 Share-based compensation 11 us-gaap_ShareBasedCompensation us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 28000000 28.0 false false 2 false true 26100000 26.1 false false No definition available. No authoritative reference available. false 19 Amortization of debt discount and debt issuance costs 11 us-gaap_AmortizationOfFinancingCostsAndDiscounts us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 5800000 5.8 false false 2 false true 5700000 5.7 false false No definition available. No authoritative reference available. false 20 Net gain on disposal or exchange of assets 11 us-gaap_GainLossOnSaleOfPropertyPlantEquipment us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -16200000 -16.2 false false 2 false true -67800000 -67.8 false false No definition available. No authoritative reference available. false 21 (Income) loss from equity affiliates 11 us-gaap_IncomeLossFromEquityMethodInvestments us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 22700000 22.7 false false 2 false true -2900000 -2.9 false false No definition available. No authoritative reference available. false 22 Revenue recovery on coal supply agreements 11 btu_RevenueRecoveryOnCoalSupplyAgreements btu false credit duration monetary Revenue recovery on coal supply agreements false false false false false false false false false 1 false true 0 0 false false 2 false true -56900000 -56.9 false false Revenue recovery on coal supply agreements No authoritative reference available. false 23 Dividends received from equity affiliates 11 us-gaap_EquityMethodInvestmentDividendsOrDistributions us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true 19900000 19.9 false false No definition available. No authoritative reference available. false 26 Changes in current assets and liabilities: 9 us-gaap_IncreaseDecreaseInReceivablesAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false No definition available. false 27 Accounts receivable, including securitization 10 us-gaap_IncreaseDecreaseInAccountsReceivable us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 43500000 43.5 false false 2 false true -149100000 -149.1 false false No definition available. No authoritative reference available. false 28 Inventories 10 us-gaap_IncreaseDecreaseInInventories us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -81000000 -81.0 false false 2 false true -10700000 -10.7 false false No definition available. No authoritative reference available. false 29 Net assets from coal trading activities 10 btu_NetAssetsFromCoalTradingActivities btu false debit duration monetary Net assets from coal trading activities false false false false false false false false false 1 false true 68800000 68.8 false false 2 false true -163100000 -163.1 false false Net assets from coal trading activities No authoritative reference available. false 30 Other current assets 10 us-gaap_IncreaseDecreaseInOtherOperatingAssets us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 15300000 15.3 false false 2 false true 5000000 5.0 false false No definition available. No authoritative reference available. false 31 Accounts payable and accrued expenses 10 us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true -146500000 -146.5 false false 2 false true 236800000 236.8 false false No definition available. No authoritative reference available. false 32 Asset retirement obligations 10 us-gaap_IncreaseDecreaseInAssetRetirementObligations us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 23200000 23.2 false false 2 false true 22900000 22.9 false false No definition available. No authoritative reference available. false 33 Workers' compensation obligations 10 us-gaap_IncreaseDecreaseInWorkersCompensationLiabilities us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 2000000 2.0 false false 2 false true 2500000 2.5 false false No definition available. No authoritative reference available. false 34 Accrued postretirement benefit costs 10 us-gaap_IncreaseDecreaseInPostretirementObligations us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 5100000 5.1 false false 2 false true -1200000 -1.2 false false No definition available. No authoritative reference available. false 35 Contributions to pension plans 10 us-gaap_PensionContributions us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -37700000 -37.7 false false 2 false true -20200000 -20.2 false false No definition available. No authoritative reference available. false 36 Distributions to noncontrolling interests 10 btu_DistributionsToNoncontrollingInterests btu false credit duration monetary DistributionsToNoncontrollingInterests false false false false false false false false false 1 false true -9300000 -9.3 false false 2 false true -1600000 -1.6 false false DistributionsToNoncontrollingInterests No authoritative reference available. false 37 Other, net 8 us-gaap_IncreaseDecreaseInOtherOperatingCapitalNet us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -9000000 -9.0 false false 2 false true -13200000 -13.2 false false No definition available. No authoritative reference available. true 38 Net cash provided by continuing operations 6 us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations us-gaap true na duration monetary No definition available. false false false false false false false false false 1 false true 664200000 664.2 false false 2 false true 798500000 798.5 false false No definition available. No authoritative reference available. false 39 Net cash used in discontinued operations 5 us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true -6200000 -6.2 false false 2 false true -106000000 -106.0 false false No definition available. No authoritative reference available. true 40 Net cash provided by operating activities 5 us-gaap_NetCashProvidedByUsedInOperatingActivities us-gaap true na duration monetary No definition available. false false false false false false false false false 1 false true 658000000 658.0 false false 2 false true 692500000 692.5 false false No definition available. No authoritative reference available. true 45 Cash Flows From Investing Activities 8 us-gaap_PaymentsToAcquirePropertyPlantAndEquipmentAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false No definition available. false 46 Additions to property, plant, equipment and mine development 9 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -143900000 -143.9 false false 2 false true -172500000 -172.5 false false No definition available. No authoritative reference available. false 47 Acquisition of noncontrolling interests (Millennium Mine) 9 us-gaap_PaymentsToAcquireAdditionalInterestInSubsidiaries us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true -106900000 -106.9 false false No definition available. No authoritative reference available. false 48 Investment in Prairie State Energy Campus 9 us-gaap_PaymentsToAcquireInterestInJointVenture us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -41600000 -41.6 false false 2 false true -28500000 -28.5 false false No definition available. No authoritative reference available. false 49 Federal coal lease expenditures 9 btu_FederalCoalLeaseExpenditures btu false debit duration monetary Federal coal lease expenditures false false false false false false false false false 1 false true -123600000 -123.6 false false 2 false true -178500000 -178.5 false false Federal coal lease expenditures No authoritative reference available. false 50 Proceeds from disposal of assets, net of notes receivable 9 us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 47500000 47.5 false false 2 false true 36300000 36.3 false false No definition available. No authoritative reference available. false 51 Additions to advance mining royalties 9 us-gaap_PaymentsToAcquireRoyaltyInterestsInMiningProperties us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -4900000 -4.9 false false 2 false true -4100000 -4.1 false false No definition available. No authoritative reference available. false 52 Investments in equity affiliates and joint ventures 9 btu_InvestmentsInEquityAffiliatesAndJointVentures btu false credit duration monetary Investments in equity affiliates and joint ventures false false false false false false false false false 1 false true -10000000 -10.0 false false 2 false true -2700000 -2.7 false false Investments in equity affiliates and joint ventures No authoritative reference available. true 53 Net cash used in continuing operations 6 us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true -276500000 -276.5 false false 2 false true -456900000 -456.9 false false No definition available. No authoritative reference available. false 54 Net cash used in discontinued operations 7 us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true -1600000 -1.6 false false No definition available. No authoritative reference available. true 55 Net cash used in investing activities 5 us-gaap_NetCashProvidedByUsedInInvestingActivities us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true -276500000 -276.5 false false 2 false true -458500000 -458.5 false false No definition available. No authoritative reference available. true 59 Cash Flows From Financing Activities 7 us-gaap_ProceedsFromRepaymentsOfShortTermDebtAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false No definition available. false 60 Change in revolving line of credit 8 us-gaap_ProceedsFromLinesOfCredit us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true -97700000 -97.7 false false No definition available. No authoritative reference available. false 61 Payments of long-term debt 8 us-gaap_RepaymentsOfLongTermDebtAndCapitalSecurities us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -11400000 -11.4 false false 2 false true -27600000 -27.6 false false No definition available. No authoritative reference available. false 62 Common stock repurchase 8 us-gaap_PaymentsForRepurchaseOfCommonStock us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true -58300000 -58.3 false false No definition available. No authoritative reference available. false 63 Dividends paid 8 us-gaap_PaymentsOfDividends us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -48100000 -48.1 false false 2 false true -48900000 -48.9 false false No definition available. No authoritative reference available. false 64 Excess tax benefit related to stock options exercised 8 us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true 27500000 27.5 false false No definition available. No authoritative reference available. false 65 Proceeds from stock options exercised 8 us-gaap_ProceedsFromStockOptionsExercised us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 1100000 1.1 false false 2 false true 13700000 13.7 false false No definition available. No authoritative reference available. false 66 Change in bank overdraft facility 8 us-gaap_ProceedsFromRepaymentsOfBankOverdrafts us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 12900000 12.9 false false 2 false true 10800000 10.8 false false No definition available. No authoritative reference available. false 67 Proceeds from employee stock purchases 8 us-gaap_ProceedsFromStockPlans us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 5100000 5.1 false false 2 false true 5200000 5.2 false false No definition available. No authoritative reference available. true 68 Net cash used in financing activities 8 us-gaap_NetCashProvidedByUsedInFinancingActivities us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true -40400000 -40.4 false false 2 false true -175300000 -175.3 false false No definition available. No authoritative reference available. true 69 Net change in cash and cash equivalents 8 us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease us-gaap true na duration monetary No definition available. false false false false false false false false false 1 false true 341100000 341.1 false false 2 false true 58700000 58.7 false false No definition available. No authoritative reference available. false 70 Cash and cash equivalents at beginning of period 8 us-gaap_CashAndCashEquivalentsAtCarryingValue us-gaap true debit instant monetary No definition available. false false false false false false true false false 1 false true 449700000 449.7 false false 2 false true 45300000 45.3 false false No definition available. No authoritative reference available. false 71 Cash and cash equivalents at end of period 3 us-gaap_CashAndCashEquivalentsAtCarryingValue us-gaap true debit instant monetary No definition available. false false false false false false false true false 1 true true 790800000 790.8 false false 2 true true 104000000 104.0 false false No definition available. No authoritative reference available. false false 2 52 false HundredThousands UnKnown UnKnown false true