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Forward Equity Sale Agreement | 12 Months Ended
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Dec. 31, 2015
Forward Equity Sale Agreements |
Forward Equity Sale Agreement | Forward Equity Sale Agreement The Company may sell shares of its common stock under a forward equity agreement. At December 31, 2015, the Company had approximately $250 million remaining notional amount that was available to sell under the forward equity agreement.

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- Definition -- None. No documentation exists for this element. -- + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Information about forward equity sale agreements, which may include amounts outstanding and capacity to sell. + References No definition available. + Details
Name: | amg_ForwardEquitySaleAgreementsTextBlock
Namespace Prefix: | amg_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration