Subsequent Events (Narrative) (Details)	Jul. 02, 2015 offer
Subsequent Events [Abstract]
Debt restructuring, number of private debt exchange offers	2

Debt Restructuring, Number of Private Debt Exchange Offers

No definition available.

Name:	aci_DebtRestructuringNumberofPrivateDebtExchangeOffers
Namespace Prefix:	aci_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant