Equity Investments And Membership Interests In Joint Ventures (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
Equity Investments And Membership Interests In Joint Ventures [Abstract]
Future development costs, maximum limit	$ 73.0

Commitments For Future Development Financing For Equity Method Investees

No definition available.

Name:	aci_CommitmentsForFutureDevelopmentFinancingForEquityMethodInvestees
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant