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Property, Plant, Equipment and Mine Development (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant, Equipment and Mine Development[Line Items]      
Land and coal interests $ 11,021,100,000btu_LandAndCoalInterests $ 11,024,100,000btu_LandAndCoalInterests  
Buildings and improvements 1,569,100,000us-gaap_BuildingsAndImprovementsGross 1,525,400,000us-gaap_BuildingsAndImprovementsGross  
Machinery and equipment 2,685,700,000us-gaap_MachineryAndEquipmentGross 2,777,500,000us-gaap_MachineryAndEquipmentGross  
Less: accumulated depreciation, depletion and amortization (4,698,600,000)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment (4,244,500,000)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment  
Total, net 10,577,300,000us-gaap_PropertyPlantAndEquipmentNet 11,082,500,000us-gaap_PropertyPlantAndEquipmentNet 11,801,700,000us-gaap_PropertyPlantAndEquipmentNet
Coal reserves 6,200,000,000btu_CoalReserves 6,300,000,000btu_CoalReserves  
Acquired interest in mineral rights 1,300,000,000btu_Acquiredinterestinmineralrights 1,300,000,000btu_Acquiredinterestinmineralrights  
Mineral rights and advanced royalties 5,000,000,000us-gaap_MineralRights 5,100,000,000us-gaap_MineralRights  
Coal reserves not subject to depletion 2,100,000,000btu_CoalReservesNotSubjectToDepletion 2,300,000,000btu_CoalReservesNotSubjectToDepletion  
Coal reserves held by fee ownership [Member]      
Property, Plant, Equipment and Mine Development[Line Items]      
Coal reserves $ 1,200,000,000btu_CoalReserves
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= btu_CoalReservesHeldByFeeOwnershipMember		 $ 1,200,000,000btu_CoalReserves
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= btu_CoalReservesHeldByFeeOwnershipMember