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