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Fair Value Measurements - Schedule of Quantitative Information (Details) (Discounted Cash Flow, USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Contingent Payment Arrangements |  |
Quantitative information for Level 3 Fair Value Measurements Liabilities |  |
Fair value Liabilities | 59.3 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByLiabilityClassAxis = amg_ContingentPaymentArrangementsMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember | 50.2 us-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 | 21.5 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember / us-gaap_ValuationTechniqueAxis = amg_DiscountedCashFlowValuationTechniqueMember | 4.0 us-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

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- Definition Represents percentage of assumed growth rates, used as an input to measure fair value. + References No definition available. + Details
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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 + Details
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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 + Details
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