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Forward Equity Sale Agreement (Details)	Dec. 31, 2015 USD ($)
Forward Equity Sale Agreements
Derivative remaining notional amount	$ 250,000,000

X

- Definition

Derivative, Remaining Notional Amount

+ References

No definition available.

+ Details

Name:	amg_DerivativeRemainingNotionalAmount
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

Name:	amg_ForwardEquitySaleAgreementsDisclosureAbstract
Namespace Prefix:	amg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration