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Forward Equity Sale Agreements (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Jul. 31, 2011 agreement
Forward equity sale agreements
Number of agreements under the forward equity sale agreements	2
Value of common stock to sell under each agreement, maximum	$ 300.0

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- Details

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

X

- Definition

Represents the maximum value of common stock under each forward equity sale agreements to sell shares of the entity's common stock.

+ References

No definition available.

+ Details

Name:	amg_ForwardEquityContractsMaximumValuePerAgreement
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Represents the number of the agreements under the forward equity sale agreements, entered by the entity.

+ References

No definition available.

+ Details

Name:	amg_ForwardEquityContractsNumberOfAgreements
Namespace Prefix:	amg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration