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Management - Labor Relations Labor Relations (Details)
 12 Months Ended
Dec. 31, 2014
Concentration Risk [Line Items]  
Entity Number of Employees 8,300dei_EntityNumberOfEmployees
Entity Number Of Hourly Employees 6,000btu_EntityNumberOfHourlyEmployees
Percentage of hourly employees represented by organized labor unions 39.00%btu_PercentageOfEmployeesRepresentedByOrganizedLaborUnions
Percentage Of Coal Production Generated By Hourly Employees Represented By Organized Labor Unions 20.00%btu_PercentageOfCoalProductionConsolidated
Number of US Mines Represented by Unions 1btu_NumberOfUsMinesRepresentedByUnions
Percentage Of Australian Hourly Employees Under Contract Negotiations 56.00%btu_PercentageOfAustralianHourlyEmployeesUnderContractNegotiations
Percentage Of Coal Production Generated By Australian Hourly Employees Under Contract Negotiation 42.00%btu_PercentageOfCoalProductionAustralia
United Mine Workers of America [Member]  
Concentration Risk [Line Items]  
Percentage of hourly employees represented by organized labor unions 9.00%btu_PercentageOfEmployeesRepresentedByOrganizedLaborUnions
/ us-gaap_ConcentrationRiskByTypeAxis
= btu_UnitedMineWorkersOfAmericaMember
Percentage Of Coal Production Generated By Hourly Employees 4.00%btu_PercentageOfCoalProductionUS
/ us-gaap_ConcentrationRiskByTypeAxis
= btu_UnitedMineWorkersOfAmericaMember