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Affiliate Equity - Summary of Affiliate Equity Expense (Details) (Affiliate Partners, USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Affiliate Partners |  |  |
Affiliate Equity |  |  |
Affiliate equity expense | $ 84.4 amg_AffiliateEquityCompensationExpenseIncludingPortionAttributabletoNoncontrollingInterest / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_AffiliatedEntityMember | $ 72.3 amg_AffiliateEquityCompensationExpenseIncludingPortionAttributabletoNoncontrollingInterest / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_AffiliatedEntityMember | $ 60.4 amg_AffiliateEquityCompensationExpenseIncludingPortionAttributabletoNoncontrollingInterest / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_AffiliatedEntityMember
Tax benefit | $ 18.2 amg_AffiliateEquityCompensationExpenseTaxBenefitExpense / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_AffiliatedEntityMember | $ 21.7 amg_AffiliateEquityCompensationExpenseTaxBenefitExpense / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_AffiliatedEntityMember | $ 17.0 amg_AffiliateEquityCompensationExpenseTaxBenefitExpense / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_AffiliatedEntityMember

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- Definition Affiliate Equity Compensation Expense, Including Portion Attributable to Noncontrolling Interest + References No definition available. + Details
Name: | amg_AffiliateEquityCompensationExpenseIncludingPortionAttributabletoNoncontrollingInterest
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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