Sale of Common Units Under Equity Distribution Agreement (Details) - USD ($) $ in Millions	3 Months Ended
Sep. 30, 2016	Sep. 30, 2018
Sale of Common Units Under Equity Distribution Agreement
Maximum aggregate offering price of common limited partners units under an equity distribution agreement	$ 250.0
Remaining capacity to issue additional common units under an equity distribution agreement		$ 157.3

Maximum aggregate offering price of common limited partners units under an equity distribution agreement.

No definition available.

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