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Affiliate Equity - Schedule of Changes in the Company's Interest in its Affiliates on the Controlling Interest's Equity (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Affiliate Equity [Abstract]    
Net income (controlling interest) $ 128.0us-gaap_NetIncomeLoss $ 77.2us-gaap_NetIncomeLoss
Increase (decrease) in controlling interest paid-in capital from purchases and sales of Affiliate equity (3.0)amg_AdjustmentsToAdditionalPaidInCapitalIncreaseDecreaseInCarryingAmountOfRedeemableNoncontrollingInterests (16.3)amg_AdjustmentsToAdditionalPaidInCapitalIncreaseDecreaseInCarryingAmountOfRedeemableNoncontrollingInterests
Change from Net income (controlling interest) and net transfers with non-controlling interests $ 125.0amg_IncreaseDecreaseInEquityDueToProfitLossAndNetTransfersWithNoncontrollingInterests $ 60.9amg_IncreaseDecreaseInEquityDueToProfitLossAndNetTransfersWithNoncontrollingInterests