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Business Combinations (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Business Acquisition |
Amortization period for tax deductible goodwill | 15 years
New Affiliates |
Business Acquisition |
New affiliate investments contributed revenue | 72.2 us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual / us-gaap_BusinessAcquisitionAxis = amg_NewAffiliatesMember
New affiliate investments contributed earnings | 8.5 us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual / us-gaap_BusinessAcquisitionAxis = amg_NewAffiliatesMember

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- Definition Business Acquisition, Goodwill, Amortization Period for Tax Deductible Amount + References No definition available. + Details
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- Definition This element represents the amount of earnings or loss of the acquiree since the acquisition date included in the consolidated income statement for the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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- Definition This element represents the amount of revenue of the acquiree since the acquisition date included in the consolidated income statement for the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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