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

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- Definition

Affiliate Equity Compensation, Tax Benefit from Compensation Expense

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No definition available.

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- Definition

Represents the compensation expenses related to Affiliate equity.

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No definition available.

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