Subsequent Events - Additional Information (Detail) (Term Loan Facility [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended	1 Months Ended
Feb. 28, 2015	Mar. 31, 2015
Subsequent Event [Line Items]
Voluntary prepayments on term loan facility	$ 75,000am_DebtInstrumentVoluntaryPrepayments
Subsequent Event [Member]
Subsequent Event [Line Items]
Voluntary prepayments on term loan facility		$ 65,000

Amount of voluntary prepayments of long-term debt.

No definition available.

Name:	am_DebtInstrumentVoluntaryPrepayments
Namespace Prefix:	am_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration