Proposal by Members of the Weiss Family and Related Entities to Acquire the Corporation - Additional Information (Detail) (Subsequent Event [Member], USD $)	1 Months Ended
Mar. 31, 2013
Subsequent Event [Member]
Pending Status of Going Private Transaction [Line Items]
Outstanding Class A and Class B common shares, price per share	$ 18.20

Offer Price Per Share to Acquire Common Shares Outstanding

No definition available.

Name:	am_OfferPricePerShareToAcquireCommonSharesOutstanding
Namespace Prefix:	am_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration