XML 57 R12.htm IDEA: XBRL DOCUMENT
v2.4.0.8

Forward Equity | 6 Months Ended
--- | ---
Jun. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract] |
Forward Equity Sales | Forward Equity Under a forward equity agreement, the Company may sell shares of common stock up to an aggregate notional amount of $400.0 million. During 2012, the Company entered into contracts to sell a notional amount of $147.2 million at an average share price of $121.37. During 2013, the Company net settled $77.0 million notional amount of forward equity contracts for cash at an average share price of $185.56. During the six months ended June 30, 2014, the Company net settled $70.2 million notional amount of forward equity contracts at an average share price of $198.71. The Company has $252.8 million remaining notional amount that it may elect to sell under the forward equity agreement.

X
- Definition Forward Equity Sales [Text Block] + References No definition available. + Details
Name: | amg_ForwardEquitySalesTextBlock
Namespace Prefix: | amg_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration