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Equity Method Investments in Affiliates (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Apr. 01, 2014
Dec. 26, 2014
Schedule of Equity Method Investments          
Amortization period for tax deductible goodwill 15 years        
Amortization expense during the period $ 122.2us-gaap_AmortizationOfIntangibleAssets $ 128.2us-gaap_AmortizationOfIntangibleAssets $ 200.0us-gaap_AmortizationOfIntangibleAssets    
Undistributed earnings from equity method investments 133.9us-gaap_RetainedEarningsUndistributedEarningsFromEquityMethodInvestees        
Amortization expense, year 1 32.0us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear        
Amortization expense, year 2 32.0us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo        
Amortization expense, year 3 32.0us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree        
Amortization expense, year 4 32.0us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour        
Amortization expense, year 5 32.0us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive        
Acquired Client Relationships Under Equity Method Investments          
Schedule of Equity Method Investments          
Weighted average life 14 years        
Amortization expense during the period 32.3us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember
41.7us-gaap_AmortizationOfIntangibleAssets
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember
     
EIG Global Energy Partners, LLC | Acquired Client Relationships Under Equity Method Investments          
Schedule of Equity Method Investments          
Definite-lived acquired client relationships       215.2us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
/ us-gaap_BusinessAcquisitionAxis
= amg_EIGGlobalEnergyPartnersLLCMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember
 
Amortization period for tax deductible goodwill       15 years  
AQR Capital Management Holdings, LLC | Acquired Client Relationships Under Equity Method Investments          
Schedule of Equity Method Investments          
Definite-lived acquired client relationships         122.2us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
/ us-gaap_BusinessAcquisitionAxis
= amg_AQRCapitalManagementHoldingsLLCMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember
Indefinite-lived acquired client relationships         $ 90.0us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets
/ us-gaap_BusinessAcquisitionAxis
= amg_AQRCapitalManagementHoldingsLLCMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember
Amortization period for tax deductible goodwill         15 years