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Affiliate Equity - Summary of Affiliate Equity Compensation (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Affiliate Equity [Abstract] |  |
Affiliate equity expense | $ 9.3 amg_CompensationExpensesRelatedToAffiliateEquity | $ 14.8 amg_CompensationExpensesRelatedToAffiliateEquity
Tax benefit | $ 1.5 amg_AffiliateEquityCompensationTaxBenefitfromCompensationExpense | $ 2.9 amg_AffiliateEquityCompensationTaxBenefitfromCompensationExpense

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- Details
Name: | amg_AffiliateEquityAbstract
Namespace Prefix: | amg_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Affiliate Equity Compensation, Tax Benefit from Compensation Expense + References No definition available. + Details
Name: | amg_AffiliateEquityCompensationTaxBenefitfromCompensationExpense
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Represents the compensation expenses related to Affiliate equity. + References No definition available. + Details
Name: | amg_CompensationExpensesRelatedToAffiliateEquity
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration