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Matters Related to the Bankruptcy Reorganization of Patriot Coal Corporation (Patriot) (Details) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 3 Months Ended 12 Months Ended 60 Months Ended
Jan. 31, 2014
Dec. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 18, 2013
Oct. 23, 2012
plaintiffs
Oct. 31, 2007
T
Matters Related to the Bankruptcy Reorganization of Patriot Coal Corporation (Patriot) [Line Items]                        
Number of company operated mines included in Patriot Spin-off                       8btu_NumberOfCompanyOperatedMinesIncludedInPatriotSpinOff
Number of majority owned JV mines included in the Patriot spin-off                       2btu_NumberOfMajorityOwnedJvMinesIncludedInPatriotSpinOff
Number of coal preparation facilities servicing contractor-operatedines included in the Patriot spin-off                       8btu_NumberOfCoalPreparationFacilitiesServicingContractorOperatedinesIncludedInPatriotSpinOff
Proven and probable coal reserves related to Patriot                       1,200,000,000btu_ProvenAndProbableCoalReservesRelatedToPatriot
Initial Payment based on the construct of the negotiated settlement $ 90.0btu_InitialPaymentbasedontheconstructofthenegotiatedsettlement                      
Amount recorded in accumulated postretirement healthcare benefit obligations to Patriot                   260.7btu_AmountrecordedinaccumulatedpostretirementhealthcarebenefitobligationstoPatriot    
NBCWA obligations included in Accumulated other comprehensive loss                   63.2btu_NBCWAobligationsincludedinAccumulatedothercomprehensivelossincome    
Credit support to Patriot   141.5btu_CreditsupporttoPatriot 122.2btu_CreditsupporttoPatriot 141.5btu_CreditsupporttoPatriot                
Fair value of credit support to Patriot   28.0btu_FairvalueofcreditsupporttoPatriot 57.6btu_FairvalueofcreditsupporttoPatriot 28.0btu_FairvalueofcreditsupporttoPatriot                
Credit support to Patriot ending after five years     85.0btu_CreditsupporttoPatriotendingafterfiveyears                  
Credit support in the form of surety bonds related to Patriots Coal Act obligations     50.7btu_CreditsupportintheformofsuretybondsrelatedtoPatriotsCoalActobligations                  
Total settlement charges   128.6btu_Settlementchargestotal   128.6btu_Settlementchargestotal                
Settlement charges related to accumulated postretirement healthcare benefit obligations of certain Patriot retirees under the NBCWA Agreement   30.6btu_SettlementchargesrelatedtothePatriotbankruptcyreorganization 0btu_SettlementchargesrelatedtothePatriotbankruptcyreorganization 30.6btu_SettlementchargesrelatedtothePatriotbankruptcyreorganization 0btu_SettlementchargesrelatedtothePatriotbankruptcyreorganization              
Charge related to settlement of other claims involving the Patriot bankruptcy   98.0btu_ChargerelatedtosettlementofotherclaimsinvolvingthePatriotbankruptcy   98.0btu_ChargerelatedtosettlementofotherclaimsinvolvingthePatriotbankruptcy                
Payment to Patriot based on the construct of the negotiated settlement 70.0btu_PaymenttoPatriotbasedontheconstructofthenegotiatedsettlement                      
Payment to the VEBA based on the construct of the negotiated settlement 20.0btu_PaymenttotheVEBAbasedontheconstructofthenegotiatedsettlement                      
Payments to the VEBA to settle the accumulated postretirement healthcare benefit obligations   240.0btu_PaymentstotheVEBAtosettletheaccumulatedpostretirementhealthcarebenefitobligations   240.0btu_PaymentstotheVEBAtosettletheaccumulatedpostretirementhealthcarebenefitobligations                
Total liabilities related to the Patriot settlement   326.1btu_TotalliabilitiesrelatedtothePatriotsettlement 271.8btu_TotalliabilitiesrelatedtothePatriotsettlement 326.1btu_TotalliabilitiesrelatedtothePatriotsettlement                
Number of individual plaintiffs involved in class action lawsuit                     8btu_NumberOfIndividualPlaintiffsInvolvedInClassActionLawsuit  
Potential exposure from Patriot bankruptcy     150.0btu_PotentialExposureFromPatriotBankruptcy                  
Scenario, Forecast [Member]                        
Matters Related to the Bankruptcy Reorganization of Patriot Coal Corporation (Patriot) [Line Items]                        
Funding of the newly established VEBA                 310.0btu_FundingofthenewlyestablishedVEBA
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		      
Payment to the VEBA based on the construct of the negotiated settlement           70.0btu_PaymenttotheVEBAbasedontheconstructofthenegotiatedsettlement
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		 75.0btu_PaymenttotheVEBAbasedontheconstructofthenegotiatedsettlement
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		 75.0btu_PaymenttotheVEBAbasedontheconstructofthenegotiatedsettlement
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember		        
Surety Bond [Member]                        
Matters Related to the Bankruptcy Reorganization of Patriot Coal Corporation (Patriot) [Line Items]                        
Credit support to Patriot     81.4btu_CreditsupporttoPatriot
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_SuretyBondMember		                  
Standby Letters of Credit [Member]                        
Matters Related to the Bankruptcy Reorganization of Patriot Coal Corporation (Patriot) [Line Items]                        
Credit support to Patriot     22.4btu_CreditsupporttoPatriot
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_StandbyLettersOfCreditMember		                  
Corporate Guarantees [Member]                        
Matters Related to the Bankruptcy Reorganization of Patriot Coal Corporation (Patriot) [Line Items]                        
Credit support to Patriot     $ 18.4btu_CreditsupporttoPatriot
/ us-gaap_GuaranteeObligationsByNatureAxis
= btu_CorporateGuaranteesMember