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Coal Trading (Details 1) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2015
Coal Trading (Textuals) [Abstract]        
Fair value hierarchy transfers from Level 1 to Level 2 $ 0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount $ 0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount    
Fair value hierarchy transfers from Level 2 to Level 1 0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount 0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount    
Coal Trading [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Derivative, Fair Value, Amount Offset Against Collateral, Net 32.6us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
[1] 56.1us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
[1]    
Changes in the Company's recurring Level 3 net financial assets        
Beginning of year 2.1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
5.2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
8.7us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
 
Total gains (realized/unrealized):        
Included in earnings 6.7us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0.3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
17.5us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
 
Settlements (6.7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlements
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
(3.4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlements
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
(21.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchasesSalesIssuancesSettlements
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
 
End of year 2.1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
2.1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
5.2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
 
Changes in unrealized gains (losses) relating to Level 3 net financial assets held both at the beginning and the end of the period        
Changes in unrealized gains (losses) 2.1us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
[2] (0.4)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
[2] 4.1us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
[2]  
Schedule of future realization of trading portfolio        
2015 77.00%btu_PercentageOfTradingPortfolioExpirationYearOne
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
2016 19.00%btu_PercentageOfTradingPortfolioExpirationYearTwo
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
2017 3.00%btu_PercentageOfTradingPortfolioExpirationYearThree
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
2018 1.00%btu_PercentageOfTradingPortfolioExpirationYearFour
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
Percentage of trading portfolio expiration, total 100.00%btu_PercentageOfTradingPortfolioExpiration
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
Coal Trading (Textuals) [Abstract]        
Cash flow hedge derivative instrument assets at fair value 44.3us-gaap_CashFlowHedgeDerivativeInstrumentAssetsAtFairValue
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
62.9us-gaap_CashFlowHedgeDerivativeInstrumentAssetsAtFairValue
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Gain on discontinuation of cash flow hedge due to forecasted transaction probable of not occurring 2.0us-gaap_GainOnDiscontinuationOfCashFlowHedgeDueToForecastedTransactionProbableOfNotOccurring
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
Fair value hierarchy transfers from Level 1 to Level 2 0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
 
Fair value hierarchy transfers from Level 2 to Level 1 0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
 
Fair value hierarchy transfers into (out of) Level 3 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
 
Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]        
Fair value coal trading net assets (liabilities) measured on recurring basis        
Commodity futures, swaps and options 32.4btu_CommoditySwapsAndOptionsFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
26.4btu_CommoditySwapsAndOptionsFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Physical commodity purchase/sale contracts (7.5)btu_PhysicalCommodityPurchaseSaleContractsFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
3.6btu_PhysicalCommodityPurchaseSaleContractsFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Total net financial assets (liabilities) 24.9btu_CoalTradingAssetsAndLiabilitiesNetFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
30.0btu_CoalTradingAssetsAndLiabilitiesNetFairValueDisclosure
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Coal Trading [Member]        
Fair value coal trading net assets (liabilities) measured on recurring basis        
Commodity futures, swaps and options (0.2)btu_CommoditySwapsAndOptionsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0btu_CommoditySwapsAndOptionsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Physical commodity purchase/sale contracts 0btu_PhysicalCommodityPurchaseSaleContractsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0btu_PhysicalCommodityPurchaseSaleContractsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Total net financial assets (liabilities) (0.2)btu_CoalTradingAssetsAndLiabilitiesNetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0btu_CoalTradingAssetsAndLiabilitiesNetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Coal Trading [Member]        
Fair value coal trading net assets (liabilities) measured on recurring basis        
Commodity futures, swaps and options 32.6btu_CommoditySwapsAndOptionsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
26.4btu_CommoditySwapsAndOptionsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Physical commodity purchase/sale contracts (9.6)btu_PhysicalCommodityPurchaseSaleContractsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
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/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
1.5btu_PhysicalCommodityPurchaseSaleContractsFairValueDisclosure
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/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Total net financial assets (liabilities) 23.0btu_CoalTradingAssetsAndLiabilitiesNetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
27.9btu_CoalTradingAssetsAndLiabilitiesNetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Coal Trading [Member]        
Fair value coal trading net assets (liabilities) measured on recurring basis        
Commodity futures, swaps and options 0btu_CommoditySwapsAndOptionsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
0btu_CommoditySwapsAndOptionsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Physical commodity purchase/sale contracts 2.1btu_PhysicalCommodityPurchaseSaleContractsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
2.1btu_PhysicalCommodityPurchaseSaleContractsFairValueDisclosure
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= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Total net financial assets (liabilities) 2.1btu_CoalTradingAssetsAndLiabilitiesNetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
2.1btu_CoalTradingAssetsAndLiabilitiesNetFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Cash Flow Hedging [Member] | Coal Trading [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Derivative, Fair Value, Amount Offset Against Collateral, Net 0us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
42.0us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
   
Minimum [Member] | Coal Trading [Member]        
Coal Trading (Textuals) [Abstract]        
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation 8.00%btu_QualityAdjustmentLevel3UnobservableInputAsPercentageOfOverallValuation
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
Location differentials Level 3 unobservable input as percentage of overall valuation 19.00%btu_LocationDifferentialsLevel3UnobservableInputAsPercentageofOverallValuation
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
Maximum [Member] | Coal Trading [Member]        
Coal Trading (Textuals) [Abstract]        
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation 12.00%btu_QualityAdjustmentLevel3UnobservableInputAsPercentageOfOverallValuation
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
Location differentials Level 3 unobservable input as percentage of overall valuation 19.00%btu_LocationDifferentialsLevel3UnobservableInputAsPercentageofOverallValuation
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
Weighted Average [Member] | Coal Trading [Member]        
Coal Trading (Textuals) [Abstract]        
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation 9.00%btu_QualityAdjustmentLevel3UnobservableInputAsPercentageOfOverallValuation
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
Location differentials Level 3 unobservable input as percentage of overall valuation 19.00%btu_LocationDifferentialsLevel3UnobservableInputAsPercentageofOverallValuation
/ us-gaap_RangeAxis
= us-gaap_WeightedAverageMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
     
Scenario, Forecast [Member] | Coal Trading [Member]        
Coal Trading (Textuals) [Abstract]        
Expected amount of gains to be realized over the next 12 months in accumulated other comprehensive loss       $ 45us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
/ us-gaap_TradingActivityByTypeAxis
= btu_CoalTradingMember
[1] None and $42 million of the net variation margin held at December 31, 2014 and 2013, respectively, related to cash flow hedges.
[2] Within the consolidated statements of operations and consolidated statements of comprehensive income for the periods presented, unrealized gains and losses from Level 3 items are combined with unrealized gains and losses on positions classified in Level 1 or 2, as well as other positions that have been realized during the applicable periods.