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Segment Information - Schedule of Segment Information (Details) (USD $) In Millions, 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
Segment Reporting Information
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
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
Total assets	7,698.1us-gaap_Assets						6,318.8us-gaap_Assets				7,698.1us-gaap_Assets	6,318.8us-gaap_Assets	6,187.1us-gaap_Assets
Goodwill	2,652.8us-gaap_Goodwill						2,341.7us-gaap_Goodwill				2,652.8us-gaap_Goodwill	2,341.7us-gaap_Goodwill	2,355.2us-gaap_Goodwill
Equity method investments in Affiliates	1,031.3us-gaap_EquityMethodInvestments						1,123.3us-gaap_EquityMethodInvestments				1,031.3us-gaap_EquityMethodInvestments	1,123.3us-gaap_EquityMethodInvestments	1,783.5us-gaap_EquityMethodInvestments
Institutional
Segment Reporting Information
Revenue											1,022.8us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	948.7us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	861.3us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Net income (controlling interest)											227.0us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	219.9us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	126.0us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Total assets	3,739.8us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember						3,196.5us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember				3,739.8us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	3,196.5us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	3,176.5us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Goodwill	1,159.1us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember						1,076.3us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember				1,159.1us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	1,076.3us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	1,078.5us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Equity method investments in Affiliates	879.6us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember						942.6us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember				879.6us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	942.6us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	1,533.8us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Mutual Fund
Segment Reporting Information
Revenue											1,242.6us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	1,023.0us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	774.4us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Net income (controlling interest)											180.1us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	103.4us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	27.3us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Total assets	3,082.0us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember						2,448.4us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember				3,082.0us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	2,448.4us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	2,354.8us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Goodwill	1,125.3us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember						928.1us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember				1,125.3us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	928.1us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	939.5us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Equity method investments in Affiliates	81.2us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember						77.7us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember				81.2us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	77.7us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	150.3us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
High Net Worth
Segment Reporting Information
Revenue											245.5us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	217.1us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	169.8us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Net income (controlling interest)											45.0us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	37.2us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	20.7us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Total assets	876.3us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember						673.9us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember				876.3us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	673.9us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	655.8us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Goodwill	368.4us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember						337.3us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember				368.4us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	337.3us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	337.2us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Equity method investments in Affiliates	$ 70.5us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember						$ 103.0us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember				$ 70.5us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	$ 103.0us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	$ 99.4us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
[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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