1.0.0.3 true Condensed Consolidated Statement of Changes in Stockholders' Equity (USD $) In Millions false 1 $ true false false false Additional Paid-in Capital us-gaap_StatementEquityComponentsAxis xbrldi http://xbrl.org/2006/xbrldi us-gaap_OtherAdditionalCapitalMember us-gaap_StatementEquityComponentsAxis explicitMember USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 2 $ true false false false Common Stock us-gaap_StatementEquityComponentsAxis xbrldi http://xbrl.org/2006/xbrldi us-gaap_CommonStockMember us-gaap_StatementEquityComponentsAxis explicitMember USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 3 $ true false false false Treasury Stock us-gaap_StatementEquityComponentsAxis xbrldi http://xbrl.org/2006/xbrldi us-gaap_TreasuryStockMember us-gaap_StatementEquityComponentsAxis explicitMember USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 4 $ true false false false Retained Earnings us-gaap_StatementEquityComponentsAxis xbrldi http://xbrl.org/2006/xbrldi us-gaap_RetainedEarningsMember us-gaap_StatementEquityComponentsAxis explicitMember USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 5 $ true false false false Accumulated Other Comprehensive Loss us-gaap_StatementEquityComponentsAxis xbrldi http://xbrl.org/2006/xbrldi us-gaap_AccumulatedOtherComprehensiveIncomeMember us-gaap_StatementEquityComponentsAxis explicitMember USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 6 $ true false false false Noncontrolling Interests us-gaap_StatementEquityComponentsAxis xbrldi http://xbrl.org/2006/xbrldi us-gaap_NoncontrollingInterestMember us-gaap_StatementEquityComponentsAxis explicitMember USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 7 $ 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 6 Balance at Dec. 31, 2008 4 us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest us-gaap true credit instant monetary No definition available. false false false true false false true false false 1 true true 2020200000 2020.2 true false 2 true true 2800000 2.8 true false 3 true true -318800000 -318.8 true false 4 true true 1802400000 1802.4 true false 5 true true -388500000 -388.5 true false 6 true true 1400000 1.4 true false 7 true true 3119500000 3119.5 false false No definition available. No authoritative reference available. false 7 Comprehensive income: 4 us-gaap_StockIssuedDuringPeriodSharebasedCompensationAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 true false 2 false false 0 0 true false 3 false false 0 0 true false 4 false false 0 0 true false 5 false false 0 0 true false 6 false false 0 0 true false 7 false false 0 0 false false No definition available. false 8 Net income 5 us-gaap_ProfitLoss us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 true false 2 false true 0 0 true false 3 false true 0 0 true false 4 false true 356000000 356.0 true false 5 false true 0 0 true false 6 false true 12000000 12.0 true false 7 false true 368000000 368.0 false false No definition available. No authoritative reference available. false 9 Increase in fair value of cash flow hedges (net of $210.5 tax provision) 5 us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodNetOfTax us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 true false 2 false true 0 0 true false 3 false true 0 0 true false 4 false true 0 0 true false 5 false true 321200000 321.2 true false 6 false true 0 0 true false 7 false true 321200000 321.2 false false No definition available. No authoritative reference available. false 10 Postretirement plans and workers' compensation obligations (net of $0.9 tax provision) 5 us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansAdjustmentNetOfTaxPeriodIncreaseDecrease us-gaap true na duration monetary No definition available. false false false false false false false false false 1 false true 0 0 true false 2 false true 0 0 true false 3 false true 0 0 true false 4 false true 0 0 true false 5 false true 1400000 1.4 true false 6 false true 0 0 true false 7 false true 1400000 1.4 false false No definition available. No authoritative reference available. true 11 Comprehensive income 5 us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecrease us-gaap true na duration monetary No definition available. false false false false false false false false false 1 false false 0 0 true false 2 false false 0 0 true false 3 false false 0 0 true false 4 false true 356000000 356.0 true false 5 false true 322600000 322.6 true false 6 false true 12000000 12.0 true false 7 false true 690600000 690.6 false false No definition available. No authoritative reference available. false 12 Dividends paid 5 us-gaap_DividendsCommonStockCash us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 true false 2 false true 0 0 true false 3 false true 0 0 true false 4 false true -48100000 -48.1 true false 5 false true 0 0 true false 6 false true 0 0 true false 7 false true -48100000 -48.1 false false No definition available. No authoritative reference available. false 13 Employee stock purchases 5 us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 5100000 5.1 true false 2 false true 0 0 true false 3 false true 0 0 true false 4 false true 0 0 true false 5 false true 0 0 true false 6 false true 0 0 true false 7 false true 5100000 5.1 false false No definition available. No authoritative reference available. false 14 Share-based compensation 5 us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 28000000 28.0 true false 2 false true 0 0 true false 3 false true 0 0 true false 4 false true 0 0 true false 5 false true 0 0 true false 6 false true 0 0 true false 7 false true 28000000 28.0 false false No definition available. No authoritative reference available. false 15 Stock options exercised 5 us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 1100000 1.1 true false 2 false true 0 0 true false 3 false true 0 0 true false 4 false true 0 0 true false 5 false true 0 0 true false 6 false true 0 0 true false 7 false true 1100000 1.1 false false No definition available. No authoritative reference available. false 16 Shares relinquished 5 btu_SharesRelinquished btu false debit duration monetary Common and/or restricted stock relinquished during the period. Recorded using the cost method. false false false false false false false false false 1 false true 0 0 true false 2 false true 0 0 true false 3 false true -2200000 -2.2 true false 4 false true 0 0 true false 5 false true 0 0 true false 6 false true 0 0 true false 7 false true -2200000 -2.2 false false Common and/or restricted stock relinquished during the period. Recorded using the cost method. No authoritative reference available. false 17 Distributions to noncontrolling interests 5 us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 true false 2 false true 0 0 true false 3 false true 0 0 true false 4 false true 0 0 true false 5 false true 0 0 true false 6 false true -9300000 -9.3 true false 7 false true -9300000 -9.3 false false No definition available. No authoritative reference available. true 22 Balance at Sep. 30, 2009 5 us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest us-gaap true credit instant monetary No definition available. false false false true false false false true false 1 true true 2054400000 2054.4 true false 2 true true 2800000 2.8 true false 3 true true -321000000 -321.0 true false 4 true true 2110300000 2110.3 true false 5 true true -65900000 -65.9 true false 6 true true 4100000 4.1 true false 7 true true 3784700000 3784.7 false false No definition available. No authoritative reference available. false false 7 13 false HundredThousands UnKnown UnKnown false true