CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Revenue	$ 635.0us-gaap_AssetManagementFees1	$ 593.1us-gaap_AssetManagementFees1
Operating expenses:
Compensation and related expenses	252.8us-gaap_LaborAndRelatedExpense	235.7us-gaap_LaborAndRelatedExpense
Selling, general and administrative	108.7	122.3
Intangible amortization and impairments	27.8us-gaap_AmortizationOfIntangibleAssets	27.4us-gaap_AmortizationOfIntangibleAssets
Depreciation and other amortization	4.4amg_DepreciationAndOtherAmortization	3.8amg_DepreciationAndOtherAmortization
Other operating expenses	9.9us-gaap_OtherCostAndExpenseOperating	9.9us-gaap_OtherCostAndExpenseOperating
Total operating expenses	403.6us-gaap_OperatingExpenses	399.1us-gaap_OperatingExpenses
Operating income	231.4us-gaap_OperatingIncomeLoss	194.0us-gaap_OperatingIncomeLoss
Income from equity method investments	53.1	46.2
Other non-operating (income) and expenses:
Investment and other income	(0.8)amg_InvestmentAndOtherIncomeLoss	(8.2)amg_InvestmentAndOtherIncomeLoss
Interest expense	22.2us-gaap_InterestExpense	17.6us-gaap_InterestExpense
Imputed interest expense and contingent payment arrangements	(27.2)	22.5
Total non-operating (income) and expenses	(5.8)us-gaap_NonoperatingIncomeExpense	31.9us-gaap_NonoperatingIncomeExpense
Income before income taxes	290.3	208.3
Income taxes	69.6us-gaap_IncomeTaxExpenseBenefit	48.9us-gaap_IncomeTaxExpenseBenefit
Net income	220.7us-gaap_ProfitLoss	159.4us-gaap_ProfitLoss
Net income (non-controlling interests)	(92.7)	(82.2)
Net income (controlling interest)	$ 128.0us-gaap_NetIncomeLoss	$ 77.2us-gaap_NetIncomeLoss
Average shares outstanding—basic (in shares)	54.8	53.7
Average shares outstanding—diluted (in shares)	57.8	55.2
Earnings per share—basic (in dollars per share)	$ 2.34us-gaap_EarningsPerShareBasic	$ 1.44us-gaap_EarningsPerShareBasic
Earnings per share—diluted (in dollars per share)	$ 2.28us-gaap_EarningsPerShareDiluted	$ 1.40us-gaap_EarningsPerShareDiluted

The aggregate expense charged against earnings to allocate the cost of tangible assets to periods in which the assets are used and held and to allocate the cost of issuing debt to periods in which the debt is outstanding. As a noncash expense, this element is added back to net income when calculating cash provided by (used in) operations using the indirect method.

No definition available.