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Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Results of Operations(Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenue	$ 641.2us-gaap_AssetManagementFees1	$ 640.3us-gaap_AssetManagementFees1	[1]	$ 636.3us-gaap_AssetManagementFees1	[1]	$ 593.1us-gaap_AssetManagementFees1	$ 594.0us-gaap_AssetManagementFees1	$ 551.6us-gaap_AssetManagementFees1	$ 541.0us-gaap_AssetManagementFees1	$ 502.2us-gaap_AssetManagementFees1	$ 2,510.9us-gaap_AssetManagementFees1	$ 2,188.8us-gaap_AssetManagementFees1	$ 1,805.5us-gaap_AssetManagementFees1
Operating income	198.7us-gaap_OperatingIncomeLoss	224.2us-gaap_OperatingIncomeLoss	[1]	198.4us-gaap_OperatingIncomeLoss	[1]	193.9us-gaap_OperatingIncomeLoss	153.1us-gaap_OperatingIncomeLoss	169.1us-gaap_OperatingIncomeLoss	161.4us-gaap_OperatingIncomeLoss	150.5us-gaap_OperatingIncomeLoss	815.2us-gaap_OperatingIncomeLoss	634.1us-gaap_OperatingIncomeLoss	400.4us-gaap_OperatingIncomeLoss
Income before income taxes	312.8us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	253.4us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	[1]	239.1us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	[1]	208.2us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	336.6us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	187.4us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	172.4us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	167.3us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	1,013.5us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	863.7us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	495.2us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Net income (controlling interest)	172.6us-gaap_NetIncomeLoss	103.2us-gaap_NetIncomeLoss	[1]	99.1us-gaap_NetIncomeLoss	[1]	77.2us-gaap_NetIncomeLoss	158.2us-gaap_NetIncomeLoss	75.2us-gaap_NetIncomeLoss	64.7us-gaap_NetIncomeLoss	62.4us-gaap_NetIncomeLoss	452.1us-gaap_NetIncomeLoss	360.5us-gaap_NetIncomeLoss	174.0us-gaap_NetIncomeLoss
Earnings per share - diluted (in dollars per share)	$ 3.02us-gaap_EarningsPerShareDiluted	$ 1.82us-gaap_EarningsPerShareDiluted	[1]	$ 1.75us-gaap_EarningsPerShareDiluted	[1]	$ 1.40us-gaap_EarningsPerShareDiluted	$ 2.79us-gaap_EarningsPerShareDiluted	$ 1.37us-gaap_EarningsPerShareDiluted	$ 1.18us-gaap_EarningsPerShareDiluted	$ 1.15us-gaap_EarningsPerShareDiluted	$ 8.01us-gaap_EarningsPerShareDiluted	$ 6.55us-gaap_EarningsPerShareDiluted	$ 3.28us-gaap_EarningsPerShareDiluted
EIG Global Energy Partners, LLC
Income before income taxes		1.7us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember	[1]	1.6us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember	[1]
Net income (controlling interest)		$ 1.1us-gaap_NetIncomeLoss / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember	[1]	$ 1.0us-gaap_NetIncomeLoss / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember	[1]
Earnings per share - diluted (in dollars per share)		$ 0.02us-gaap_EarningsPerShareDiluted / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember		$ 0.02us-gaap_EarningsPerShareDiluted / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember	[1]
[1] In the fourth quarter of 2014, the Company finalized its purchase price allocation for its investment in EIG. The final purchase price allocation resulted in a change in Income from equity method investments and Intangible-related deferred taxes in the second and third quarters of 2014. Income before taxes decreased $1.6 million and $1.7 million in the second and third quarters, respectively, Net income (controlling interest) decreased $1.0 million and $1.1 million in the second and third quarters, respectively, and Earnings per share (diluted) decreased $0.02 in both periods.

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