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Share-Based Compensation - Restricted Stock (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award      
Weighted average period for recognition (in years) 3 years    
Restricted stock available for future grant under the Company's option plans (in shares) 1.7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant    
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award      
Award fair value $ 8.0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
$ 34.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
$ 18.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Vesting period   6 years  
Market condition 1 percentage   15.00%amg_SharebasedCompensationArrangementbySharebasedPaymentAwardMarketCondition1Percent
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Market condition 2 percentage   25.00%amg_SharebasedPaymentAwardMarketCondition2RequisitePercentageIncreaseinStockPrice
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Market condition 3 percentage   35.00%amg_SharebasedPaymentAwardMarketCondition3RequisitePercentageIncreaseinStockPrice
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Expected volatility   31.60%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Risk-free interest rate   2.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Dividend yield   0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Fair value of restricted stock awards   $ 19.4amg_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsGrantsinPeriodAggregateIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Weighted average period for recognition (in years)   4 years  
Restricted Stock | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award      
Vesting period 3 years    
Restricted Stock | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award      
Vesting period 8 years