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Derivatives (Effects of Derivatives on Measures of Financial Performance) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (Loss) Recognized in OCI (Effective Portion) $ 5,745us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet $ 188us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet $ 5,250us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
Gains (Losses) Reclassified from OCI into Income (Effective Portion) (2,643)us-gaap_CostOfGoodsSold (2,981)us-gaap_CostOfGoodsSold (2,675)us-gaap_CostOfGoodsSold
Coal Sales [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (Loss) Recognized in OCI (Effective Portion) 10,842us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalSalesMember		 [1] (338)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalSalesMember		 [1] 7,690us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalSalesMember
Gains (Losses) Reclassified from OCI into Income (Effective Portion) (5,336)us-gaap_CostOfGoodsSold
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalSalesMember		 [1] (3,664)us-gaap_CostOfGoodsSold
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalSalesMember		 [1] (2,675)us-gaap_CostOfGoodsSold
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalSalesMember
Coal Purchases [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (Loss) Recognized in OCI (Effective Portion) (5,097)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalPurchasesMember		 [2] 526us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalPurchasesMember		 [2] (2,440)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalPurchasesMember
Gains (Losses) Reclassified from OCI into Income (Effective Portion) (2,693)us-gaap_CostOfGoodsSold
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalPurchasesMember		 [2] (683)us-gaap_CostOfGoodsSold
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalPurchasesMember		 [2] 0us-gaap_CostOfGoodsSold
/ us-gaap_IncomeStatementLocationAxis
= aci_CoalPurchasesMember
Coal [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Unrealized Gain (Loss) on Derivatives 430us-gaap_UnrealizedGainLossOnDerivatives
/ us-gaap_TradingActivityByTypeAxis
= us-gaap_PublicUtilitiesInventoryCoalMember		 [3] (12,700)us-gaap_UnrealizedGainLossOnDerivatives
/ us-gaap_TradingActivityByTypeAxis
= us-gaap_PublicUtilitiesInventoryCoalMember		 [3] 8,272us-gaap_UnrealizedGainLossOnDerivatives
/ us-gaap_TradingActivityByTypeAxis
= us-gaap_PublicUtilitiesInventoryCoalMember
Coal derivative settlements, non-hedging 5,956us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_TradingActivityByTypeAxis
= us-gaap_PublicUtilitiesInventoryCoalMember		 [4] 32,534us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_TradingActivityByTypeAxis
= us-gaap_PublicUtilitiesInventoryCoalMember		 [4] 43,990us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_TradingActivityByTypeAxis
= us-gaap_PublicUtilitiesInventoryCoalMember
Heating Oil-Diesel Purchases [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Realized Gains (Losses) (7,848)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_TradingActivityByTypeAxis
= aci_HeatingOilDieselPurchasesMember		 [4] (9,791)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_TradingActivityByTypeAxis
= aci_HeatingOilDieselPurchasesMember		 [4] (22,281)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_TradingActivityByTypeAxis
= aci_HeatingOilDieselPurchasesMember
Heating Oil-Fuel Surchages [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Realized Gains (Losses) $ (405)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_TradingActivityByTypeAxis
= aci_HeatingOilFuelSurchagesMember		 [4] $ (947)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_TradingActivityByTypeAxis
= aci_HeatingOilFuelSurchagesMember		 [4] $ (2,209)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_TradingActivityByTypeAxis
= aci_HeatingOilFuelSurchagesMember
[1] Revenues
[2] Cost of sales
[3] Change in fair value of coal derivatives and coal trading activities, net
[4] Other operating income, net