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

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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
Period Type: | duration

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

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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