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Forward Equity Sale Agreements (Details) (Amended and restated forward equity sale agreement, USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended		3 Months Ended
	Oct. 31, 2012	Aug. 31, 2012	Sep. 30, 2012
Amended and restated forward equity sale agreement
Forward equity sale agreements
Value of common stock to sell under each agreement, maximum		$ 400.0
Notional amount sold	$ 10.4		$ 136.8
Average share price (in dollars per share)	$ 123.88		$ 121.19

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- Details

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- 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
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- Definition

Represents the sum of common stock sold under the forward equity sale agreements.

+ References

No definition available.

+ Details

Name:	amg_ForwardEquityContractsSharesSoldAmount
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Represents the average share price of common stock sold under the forward equity sale agreements.

+ References

No definition available.

+ Details

Name:	amg_ForwardEquityContractsSharesSoldAverageSharePrice
Namespace Prefix:	amg_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration