CONSOLIDATED STATEMENTS OF INCOME (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenue	$ 2,510.9us-gaap_AssetManagementFees1	$ 2,188.8us-gaap_AssetManagementFees1	$ 1,805.5us-gaap_AssetManagementFees1
Operating expenses:
Compensation and related expenses	1,030.5us-gaap_LaborAndRelatedExpense	947.5us-gaap_LaborAndRelatedExpense	784.7us-gaap_LaborAndRelatedExpense
Selling, general and administrative	485.5	427.2	366.9
Intangible amortization and impairments	122.2us-gaap_AmortizationOfIntangibleAssets	128.2us-gaap_AmortizationOfIntangibleAssets	200.0us-gaap_AmortizationOfIntangibleAssets
Depreciation and other amortization	16.9amg_DepreciationAndOtherAmortization	14.0amg_DepreciationAndOtherAmortization	14.1amg_DepreciationAndOtherAmortization
Other operating expenses	40.6us-gaap_OtherCostAndExpenseOperating	37.8us-gaap_OtherCostAndExpenseOperating	39.4us-gaap_OtherCostAndExpenseOperating
Total operating expenses	1,695.7us-gaap_OperatingExpenses	1,554.7us-gaap_OperatingExpenses	1,405.1us-gaap_OperatingExpenses
Operating income	815.2us-gaap_OperatingIncomeLoss	634.1us-gaap_OperatingIncomeLoss	400.4us-gaap_OperatingIncomeLoss
Income from equity method investments	281.7	307.8	129.7
Other non-operating (income) and expenses:
Investment and other income	(23.3)amg_InvestmentAndOtherIncomeLoss	(40.8)amg_InvestmentAndOtherIncomeLoss	(22.0)amg_InvestmentAndOtherIncomeLoss
Interest expense	76.6us-gaap_InterestExpense	87.3us-gaap_InterestExpense	83.0us-gaap_InterestExpense
Imputed interest expense and contingent payment arrangements	30.1amg_ImputedInterest	31.7amg_ImputedInterest	(26.1)amg_ImputedInterest
Total non-operating (income) and expenses	83.4us-gaap_NonoperatingIncomeExpense	78.2us-gaap_NonoperatingIncomeExpense	34.9us-gaap_NonoperatingIncomeExpense
Income before income taxes	1,013.5	863.7	495.2
Income taxes	227.9us-gaap_IncomeTaxExpenseBenefit	194.1us-gaap_IncomeTaxExpenseBenefit	83.8us-gaap_IncomeTaxExpenseBenefit
Net income	785.6us-gaap_ProfitLoss	669.6us-gaap_ProfitLoss	411.4us-gaap_ProfitLoss
Net income (non-controlling interests)	(333.5)	(309.1)	(237.4)
Net income (controlling interest)	$ 452.1us-gaap_NetIncomeLoss	$ 360.5us-gaap_NetIncomeLoss	$ 174.0us-gaap_NetIncomeLoss
Earnings per share - basic (in dollars per share)	$ 8.22us-gaap_EarningsPerShareBasic	$ 6.79us-gaap_EarningsPerShareBasic	$ 3.36us-gaap_EarningsPerShareBasic
Earnings per share - diluted (in dollars per share)	$ 8.01us-gaap_EarningsPerShareDiluted	$ 6.55us-gaap_EarningsPerShareDiluted	$ 3.28us-gaap_EarningsPerShareDiluted
Average shares outstanding - basic (in shares)	55.0	53.1	51.7
Average shares outstanding - diluted (in shares)	58.4	56.7	53.0

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.