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Fair Value Measurements - Schedule of Quantitative Information (Details) (Discounted Cash Flow, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Contingent Payment Arrangements
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	59.3us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	50.2us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember
Contingent Payment Arrangements | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	6.00%amg_FairValueInputsGrowthRates / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	3.00%amg_FairValueInputsGrowthRates / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember
Discount rate (as a percent)	15.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	14.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember
Contingent Payment Arrangements | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	6.00%amg_FairValueInputsGrowthRates / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	11.00%amg_FairValueInputsGrowthRates / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember
Discount rate (as a percent)	15.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	18.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember
Affiliate Equity Repurchase Obligations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	21.5us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	4.0us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember
Affiliate Equity Repurchase Obligations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	5.00%amg_FairValueInputsGrowthRates / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	8.00%amg_FairValueInputsGrowthRates / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember
Discount rate (as a percent)	15.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	15.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember
Affiliate Equity Repurchase Obligations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	9.00%amg_FairValueInputsGrowthRates / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	8.00%amg_FairValueInputsGrowthRates / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember
Discount rate (as a percent)	16.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember	15.00%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember

X

- Definition

Represents percentage of assumed growth rates, used as an input to measure fair value.

+ References

No definition available.

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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