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Forward Equity Sale Agreements (Details) (USD $) In Millions, unless otherwise specified | 1 Months Ended
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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

X
- 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