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

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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

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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.

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-Name Accounting Standards Codification

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