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Asset Retirement Obligations (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
asset retirement obligations [Line Items]  
Letters of Credit Outstanding, Amount $ 3.5us-gaap_LettersOfCreditOutstandingAmount
Asset retirement obligations [Member]  
asset retirement obligations [Line Items]  
Self-bonding obligations 458.5aci_Selfbondingamounts
/ us-gaap_BalanceSheetLocationAxis
= aci_AssetRetirementObligationsMember
Asset retirement obligations [Member]  
asset retirement obligations [Line Items]  
Surety bonds outstanding $ 177.7aci_Suretybondsoutstanding
/ us-gaap_RegulatoryLiabilityAxis
= aci_AssetRetirementObligationsMember