Workers' Compensation Expense (Narrative) (Details) (Workers' Compensation Benefits [Domain], USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014
Workers' Compensation Benefits [Domain]
Workers' Compensation [Line Items]
Surety Bonds And Letters Of Credit Outstanding	$ 121.4

Surety bonds and letters of credit outstanding for future obligaitions.

No definition available.

Name:	aci_Suretybondsandlettersofcreditoutstanding
Namespace Prefix:	aci_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration