Equity Investments And Membership Interests In Joint Ventures (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Equity Method Investments and Joint Ventures [Abstract]
Future development costs, maximum limit	$ 72.9

Commitments For Future Development Financing For Equity Method Investees

No definition available.

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