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Stock-Based Compensation and Other Incentive Plans (Restricted Stock and Restricted Stock Unit Activity and Weighted Average Grant-Date Fair Value) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
12 Months Ended 3 Months Ended
Dec. 31, 2014
Dec. 31, 2014
Restricted Stock [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]    
Oustanding as of January 1 143us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
 
Shares granted during period 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
 
Vested (120)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
 
Canceled (1)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
 
Outstanding as of December 31 22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
Outstanding Weighted Average Grant Date Price $ 13.44us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
13.44us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
Weighted average fair value of restricted stock granted $ 0.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
 
Vested, Weighted Average Grant-Date Fair Value $ 30.08us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
 
Canceled, Weighted Average Grant-Date Fair Value $ 32.49aci_Sharebasedcompensationarrangementbysharebasedpaymentawardunvestedoptionsforfeituresinperiodweightedaveragegrantdatefairvalue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
 
Outstanding Weighted Average Grant Date Price $ 27.52us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockMember
 
Restricted Stock Units (RSUs) [Member]    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]    
Oustanding as of January 1 1,373us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Shares granted during period 1,521us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Vested 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Canceled (70)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Outstanding as of December 31 2,824us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
2,824us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
Outstanding Weighted Average Grant Date Price $ 6.14us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
6.14us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
Weighted average fair value of restricted stock granted $ 4.51us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Vested, Weighted Average Grant-Date Fair Value $ 0.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Canceled, Weighted Average Grant-Date Fair Value $ 6.34aci_Sharebasedcompensationarrangementbysharebasedpaymentawardunvestedoptionsforfeituresinperiodweightedaveragegrantdatefairvalue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Outstanding Weighted Average Grant Date Price $ 7.95us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
/ us-gaap_StatementClassOfStockAxis
= us-gaap_RestrictedStockUnitsRSUMember
 
Restricted Stock Units (RSUs) [Member]    
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period   3 years