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Segment Information - Schedule of Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 Channel Segment	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting [Abstract]
Number of operating segments	3us-gaap_NumberOfReportableSegments
Number of principal distribution channels	3amg_NumberOfPrincipalDistributionChannels
Statements of Income
Revenue	$ 635.0us-gaap_AssetManagementFees1	$ 593.1us-gaap_AssetManagementFees1
Net income (controlling interest)	128.0us-gaap_NetIncomeLoss	77.2us-gaap_NetIncomeLoss
Balance Sheet Information
Total Assets	7,493.9us-gaap_Assets		7,698.1us-gaap_Assets
Institutional
Statements of Income
Revenue	252.9us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	244.9us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Net income (controlling interest)	48.7us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember	35.9us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Balance Sheet Information
Total Assets	3,417.2us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember		3,739.8us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_InstitutionalMember
Mutual Fund
Statements of Income
Revenue	318.3us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	291.9us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Net income (controlling interest)	67.6us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember	34.1us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
Balance Sheet Information
Total Assets	3,253.9us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember		3,082.0us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_MutualFundMember
High Net Worth
Statements of Income
Revenue	63.8us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	56.3us-gaap_AssetManagementFees1 / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Net income (controlling interest)	11.7us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember	7.2us-gaap_NetIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember
Balance Sheet Information
Total Assets	$ 822.8us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember		$ 876.3us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = amg_HighNetWorthMember

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