Affiliate Equity - Summary of Affiliate Equity Compensation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Affiliate Equity [Abstract]
Affiliate equity expense	$ 14.1	$ 7.9	$ 60.0	$ 31.6
Tax benefit	$ 2.4	$ 2.2	$ 9.6	$ 7.8

Affiliate Equity Compensation, Tax Benefit from Compensation Expense

No definition available.

Name:	amg_AffiliateEquityCompensationTaxBenefitfromCompensationExpense
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration