Deferred Costs - Additional Information (Detail) - USD ($) $ in Millions	Nov. 25, 2016	Feb. 29, 2016	Nov. 27, 2015
Deferred Costs [Abstract]
Allowance for deferred costs related to supply agreements	$ 3.1	$ 3.6	$ 3.6

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