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Fair Value Measurements - Summary of Investments in Certain Entities that Calculate Net Asset Value (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
NAV of investments at fair value    
Fair value $ 221.6us-gaap_AlternativeInvestmentsFairValueDisclosure $ 210.0us-gaap_AlternativeInvestmentsFairValueDisclosure
Unfunded commitments 66.4us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments 67.8us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
Life of funds (in years) 15 years  
Private Equity Funds    
NAV of investments at fair value    
Fair value 133.8us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember		 [1] 134.2us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember		 [1]
Unfunded commitments 66.4us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember		 [1] 67.8us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember		 [1]
Other Funds    
NAV of investments at fair value    
Fair value 87.8us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= amg_OtherFundsMember		 [2] 75.8us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= amg_OtherFundsMember		 [2]
Unfunded commitments $ 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= amg_OtherFundsMember		 [2] $ 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= amg_OtherFundsMember		 [2]
[1] These funds primarily invest in a broad range of private equity funds, as well as make direct investments. Distributions will be received as the underlying assets are liquidated over the life of the funds, which is generally up to 15 years.
[2] These are multi-disciplinary funds that invest across various asset classes and strategies, including long/short equity, credit and real estate. Investments are generally redeemable on a daily or quarterly basis.