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Affiliate Equity (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Affiliate Equity
Affiliate equity compensation expense attributable to noncontrolling interest	$ 37.0amg_AffiliateEquityCompensationExpenseAttributabletoNoncontrollingInterest	$ 15.9amg_AffiliateEquityCompensationExpenseAttributabletoNoncontrollingInterest	$ 16.3amg_AffiliateEquityCompensationExpenseAttributabletoNoncontrollingInterest
Compensation expenses related to Affiliate equity	71.1amg_CompensationExpensesRelatedToAffiliateEquity	68.2amg_CompensationExpensesRelatedToAffiliateEquity
Weighted average period of recognition	4 years
Compensation expenses related to Affiliate equity, attributable to controlling interest	$ 41.6amg_CompensationExpensesRelatedToAffiliateEquityAttributableToControllingInterest	$ 32.1amg_CompensationExpensesRelatedToAffiliateEquityAttributableToControllingInterest

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- Definition

Affiliate Equity Compensation Expense, Attributable to Noncontrolling Interest

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- Definition

Affiliate Equity Compensation Expense, Weighted Average Period of Recognition

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- Definition

Represents the compensation expenses related to Affiliate equity.

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- Definition

Represents the compensation expenses related to affiliate equity, which were attributable to the controlling interest.

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No definition available.

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