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Fair Value Measurements - Schedule of Quantitative Information used in Valuing Level 3 Liabilities (Details) (Discounted Cash Flow, USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Contingent Payment Arrangements
    
Quantitative information for Level 3 Fair Value Measurements Liabilities    
Fair value Liabilities 31.5us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember		 59.3us-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) 5.00%amg_FairValueInputsGrowthRates
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember		 6.00%amg_FairValueInputsGrowthRates
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember
Discount rate (as a percent) 12.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember		 15.00%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) 5.00%amg_FairValueInputsGrowthRates
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember		 6.00%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		 15.00%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 16.8us-gaap_LiabilitiesFairValueDisclosure
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember		 21.5us-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) 1.00%amg_FairValueInputsGrowthRates
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember		 5.00%amg_FairValueInputsGrowthRates
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember
Discount rate (as a percent) 12.00%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) 10.00%amg_FairValueInputsGrowthRates
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember		 9.00%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		 16.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= amg_DiscountedCashFlowValuationTechniqueMember