Deferred Costs - Additional Information (Detail) - USD ($) $ in Millions	Aug. 28, 2015	Feb. 28, 2015	Aug. 29, 2014
Deferred Costs [Abstract]
Allowance for deferred costs related to supply agreements	$ 3.5	$ 2.3	$ 3.1

Allowance included in other assets for deferred costs related to retail supply agreements.

No definition available.

Name:	am_AllowanceForDeferredCosts
Namespace Prefix:	am_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant