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Derivatives and Fair Value Measurements (Details 1)
In Millions, unless otherwise specified
 12 Months Ended 12 Months Ended
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2012
USD ($)
Dec. 31, 2014
Gains realized on previously monetized foreign currency cash flow hedges [Member]
USD ($)
Dec. 31, 2014
Commodity swap contracts [Member]
Operating costs and expenses [Member]
Cash flow hedges [Member]
USD ($)
Dec. 31, 2013
Commodity swap contracts [Member]
Operating costs and expenses [Member]
Cash flow hedges [Member]
USD ($)
Dec. 31, 2012
Commodity swap contracts [Member]
Operating costs and expenses [Member]
Cash flow hedges [Member]
USD ($)
Dec. 31, 2014
Foreign currency forward contracts [Member]
AUD
Dec. 31, 2014
Foreign currency forward contracts [Member]
Fair value hedge [Member]
AUD
Dec. 31, 2014
Foreign currency forward contracts [Member]
Cash flow hedges [Member]
AUD
Dec. 31, 2014
Foreign currency forward contracts [Member]
Economic hedge [Member]
AUD
Dec. 31, 2014
Foreign currency forward contracts [Member]
Operating costs and expenses [Member]
Cash flow hedges [Member]
USD ($)
Dec. 31, 2013
Foreign currency forward contracts [Member]
Operating costs and expenses [Member]
Cash flow hedges [Member]
USD ($)
Dec. 31, 2012
Foreign currency forward contracts [Member]
Operating costs and expenses [Member]
Cash flow hedges [Member]
USD ($)
Derivative Instruments, Gain (Loss) [Line Items]                            
Monetization of foreign currency hedge positions $ 0btu_MonetizationOfForeignCurrencyHedgePositions $ 0btu_MonetizationOfForeignCurrencyHedgePositions $ 151.8btu_MonetizationOfForeignCurrencyHedgePositions                      
Derivative, Notional Amount               3,191.1invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember		 0invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember		 3,191.1invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember		 0invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= btu_EconomicHedgeMember		      
Gain (loss) recognized in other comprehensive income on derivative (effective portion) (295.4)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet (585.3)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet 162.5us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet   (194.5)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 12.5us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 14.5us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		         (100.9)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 (597.8)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 148.0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember
Gain (loss) reclassified from other comprehensive income into income (effective portion) (47.9)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet [1] 174.3us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet 400.0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet   (20.6)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 11.9us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 48.3us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		         (27.3)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 [1] 162.4us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 351.7us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember
Gain (loss) reclassified from other comprehensive income into income (ineffective portion) (1.7)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet (0.5)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet (4.7)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet   (1.7)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 (0.5)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 (4.7)us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		         0us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 0us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember		 0us-gaap_DerivativeInstrumentsGainLossRecognizedInIncomeIneffectivePortionAndAmountExcludedFromEffectivenessTestingNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_IncomeStatementLocationAxis
= btu_OperatingCostsAndExpensesMember
Gain reclassified to earnings for previously monetized foreign currency forward contracts       $ 136.9us-gaap_ForeignCurrencyCashFlowHedgeGainLossReclassifiedToEarningsNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= btu_GainsrealizedonpreviouslymonetizedforeigncurrencycashflowhedgesMember		                    
[1] Includes the reclassification from "Accumulated other comprehensive loss" into earnings of $136.9 million of previously unrecognized gains on foreign currency cash flow hedge contracts monetized in the fourth quarter of 2012.