Affiliated Managers (MGR) 10-K 23 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 18, 2015	Jun. 30, 2014
Document and Entity Information
Entity Registrant Name	AFFILIATED MANAGERS GROUP, INC.
Entity Central Index Key	1004434
Document Type	10-K
Document Period End Date	31-Dec-14
Amendment Flag	FALSE
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$11,317,162,064
Entity Common Stock, Shares Outstanding		54,895,148
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY

CONSOLIDATED_STATEMENTS_OF_INC

CONSOLIDATED STATEMENTS OF INCOME (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenue	$2,510.90	$2,188.80	$1,805.50
Operating expenses:
Compensation and related expenses	1,030.50	947.5	784.7
Selling, general and administrative	485.5	427.2	366.9
Intangible amortization and impairments	122.2	128.2	200
Depreciation and other amortization	16.9	14	14.1
Other operating expenses	40.6	37.8	39.4
Total operating expenses	1,695.70	1,554.70	1,405.10
Operating income	815.2	634.1	400.4
Income from equity method investments	281.7	307.8	129.7
Other non-operating (income) and expenses:
Investment and other income	-23.3	-40.8	-22
Interest expense	76.6	87.3	83
Imputed interest expense and contingent payment arrangements	30.1	31.7	-26.1
Total non-operating (income) and expenses	83.4	78.2	34.9
Income before income taxes	1,013.50	863.7	495.2
Income taxes	227.9	194.1	83.8
Net income	785.6	669.6	411.4
Net income (non-controlling interests)	-333.5	-309.1	-237.4
Net income (controlling interest)	$452.10	$360.50	$174
Earnings per share - basic (in dollars per share)	$8.22	$6.79	$3.36
Earnings per share - diluted (in dollars per share)	$8.01	$6.55	$3.28
Average shares outstanding - basic (in shares)	55	53.1	51.7
Average shares outstanding - diluted (in shares)	58.4	56.7	53

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$785.60	$669.60	$411.40
Other comprehensive income (loss):
Foreign currency translation adjustment	-62	-19.6	18.5
Change in net realized and unrealized gain (loss) on derivative securities, net of tax	0.3	1	-0.7
Change in net unrealized gain (loss) on investment securities, net of tax	3.4	11.5	13.5
Other comprehensive income (loss)	-58.3	-7.1	31.3
Comprehensive income	727.3	662.5	442.7
Comprehensive income (non-controlling interests)	-317.4	-311.1	-239.6
Comprehensive income (controlling interest)	$409.90	$351.40	$203.10

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Assets
Cash and cash equivalents	$550.60	$469.60
Receivables	425.9	418.4
Investments in marketable securities	172.6	157.9
Other investments	167.2	164.3
Fixed assets, net	95.4	92.3
Goodwill	2,652.80	2,341.70
Acquired client relationships, net	1,778.40	1,460.70
Equity method investments in Affiliates	1,783.50	1,123.30
Other assets	71.7	90.6
Total assets	7,698.10	6,318.80
Liabilities and Equity
Payables and accrued liabilities	808.3	514.7
Senior bank debt	855	525
Senior notes	736.8	340
Convertible securities	303.1	518.7
Deferred income taxes	491.7	456.9
Other liabilities	214.5	177
Total liabilities	3,409.40	2,532.30
Commitments and contingencies (Note 10)
Redeemable non-controlling interests	645.5	641.9
Equity:
Common stock ($.01 par value, 153.0 shares authorized; 53.9 shares outstanding in 2013 and 55.8 shares outstanding in 2014)	0.6	0.5
Additional paid-in capital	672.2	479.9
Accumulated other comprehensive income	31.8	74
Retained earnings	2,163.30	1,711.20
Total stockholders' equity before treasury stock	2,867.90	2,265.60
Less: Treasury stock, at cost (1.0 shares in 2013 and 1.2 shares in 2014)	-240.9	-131.4
Total stockholders' equity	2,627	2,134.20
Non-controlling interests	1,016.20	1,010.40
Total equity	3,643.20	3,144.60
Total liabilities and equity	$7,698.10	$6,318.80

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, except Per Share data, unless otherwise specified
Statement of Financial Position [Abstract]
Common stock, par value (in dollars per share)	$0.01	$0.01
Common stock, shares authorized	153	153
Common stock, shares outstanding	55.8	53.9
Treasury stock, shares	1.2	1

CONSOLIDATED_STATEMENTS_OF_CHA

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $)	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock at Cost	Non-Controlling Interests
In Millions, unless otherwise specified
Beginning Balance at Dec. 31, 2011	$2,499.60	$0.50	$927.50	$50	$1,176.70	($288.70)	$633.60
Beginning balance (in shares) at Dec. 31, 2011	53.9
Increase (Decrease) in Stockholders' Equity
Net income	411.4				174		237.4
Share-based compensation	25.4		25.4
Common stock issued under share-based incentive plans	73		-62			135
Tax benefit from share-based incentive plans	25.5		25.5
Affiliate equity activity	-26		-47.9				21.9
Investments in Affiliates	247.6						247.6
Shares repurchases	-60.9					-60.9
Distributions to non-controlling interests	-185.5						-185.5
Other comprehensive income (loss)	31.3			29.1			2.2
Ending Balance at Dec. 31, 2012	3,041.40	0.5	868.5	79.1	1,350.70	-214.6	957.2
Ending balance (in shares) at Dec. 31, 2012	53.9
Increase (Decrease) in Stockholders' Equity
Net income	669.6				360.5		309.1
Share-based compensation	27.5		27.5
Common stock issued under share-based incentive plans	45.7		-53.2			98.9
Tax benefit from share-based incentive plans	20.7		20.7
Settlement of convertible securities	-130.7		-130.7
Forward equity	-44		-44
Affiliate equity activity	-195.7		-208.9				13.2
Shares repurchases	-15.7					-15.7
Distributions to non-controlling interests	-267.1						-267.1
Other comprehensive income (loss)	-7.1			-5.1			-2
Ending Balance at Dec. 31, 2013	3,144.60	0.5	479.9	74	1,711.20	-131.4	1,010.40
Ending balance (in shares) at Dec. 31, 2013	53.9
Increase (Decrease) in Stockholders' Equity
Net income	785.6				452.1		333.5
Share-based compensation	29.3		29.3
Common stock issued under share-based incentive plans	-5.3		-134.4			129.1
Tax benefit from share-based incentive plans	60.2		60.2
Settlement of convertible securities (in shares)	1.9
Settlement of convertible securities	276.5	0.1	276.4
Forward equity	-45		-45
Affiliate equity activity	28.6		5.8				22.8
Investments in Affiliates	235						235
Shares repurchases	-238.6					-238.6
Distributions to non-controlling interests	-569.4						-569.4
Other comprehensive income (loss)	-58.3			-42.2			-16.1
Ending Balance at Dec. 31, 2014	$3,643.20	$0.60	$672.20	$31.80	$2,163.30	($240.90)	$1,016.20
Ending balance (in shares) at Dec. 31, 2014	55.8

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flow from (used in) operating activities:
Net income	$785.60	$669.60	$411.40
Adjustments to reconcile Net income to net cash flow from operating activities:
Intangible amortization and impairments	122.2	128.2	200
Depreciation and other amortization	16.9	14	14.1
Deferred income tax provision	62.8	27.7	11.1
Imputed interest expense and contingent payment arrangements	30.1	31.7	-26.1
Income from equity method investments, net of amortization	-281.7	-307.8	-129.7
Distributions received from equity method investments	366.9	226.6	104.7
Share-based compensation and Affiliate equity expense	113.7	84.1	69.3
Other non-cash items	11.4	20	24.3
Changes in assets and liabilities:
(Increase) decrease in receivables	26.5	-101.8	-41.6
Increase in other assets	-7.1	-12.8	-4.7
Increase in payables, accrued liabilities and other liabilities	144.9	177.6	0.4
Cash flow from operating activities	1,392.20	957.1	633.2
Cash flow from (used in) investing activities:
Investments in Affiliates	-1,245	-26.3	-797.4
Purchase of fixed assets	-19.2	-24	-20
Purchase of investment securities	-21.2	-11.4	-19.1
Sale of investment securities	17.3	11.4	34.2
Cash flow used in investing activities	-1,268.10	-50.3	-802.3
Cash flow from (used in) financing activities:
Borrowings of senior debt	1,746.50	760	970
Repayments of senior debt and convertible securities	-1,020.60	-1,201.30	-555
Issuance of common stock	41.4	48.2	73.4
Repurchase of common stock	-190.8	-15.7	-60.9
Note and contingent payments	14.4	-41	-3.6
Distributions to non-controlling interests	-569.4	-267.1	-181.4
Affiliate equity issuances and repurchases	-65.7	-118.1	-107.9
Excess tax benefit from share-based compensation	61.5	17.3	22
Settlement of forward equity sale agreement	-45	-44	0
Other financing items	-5.2	-7.4	-10.4
Cash flow from (used in) financing activities	-32.9	-869.1	146.2
Effect of foreign exchange rate changes on cash and cash equivalents	-10.2	1.5	3.8
Net increase (decrease) in cash and cash equivalents	81	39.2	-19.1
Cash and cash equivalents at beginning of period	469.6	430.4	449.5
Cash and cash equivalents at end of period	550.6	469.6	430.4
Supplemental disclosure of cash flow information:
Interest paid	67.9	87.4	79.8
Income taxes paid	110.7	82.8	51.6
Supplemental disclosure of non-cash financing activities:
Settlement of 2006 junior convertible securities	217.8	0	0
Stock issued under other incentive plans	63.6	1.3	0.5
Stock received in settlement of liability	44.7	0.5	0.1
Payables recorded for Share repurchases	47.8	0	0
Payables recorded for Affiliate equity repurchases	21.5	4	10.6
Payables recorded under contingent payment arrangements	$0	$0	$28.90

Business_and_Summary_of_Signif

Business and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Business and Summary of Significant Accounting Policies	Business and Summary of Significant Accounting Policies
	(a)	Organization and Nature of Operations
	Affiliated Managers Group, Inc. ("AMG" or the "Company") is a global asset management company with equity investments in a diverse group of boutique investment management firms ("Affiliates"). AMG's Affiliates provide investment management services globally to institutional clients, mutual funds and high net worth individuals.
	Affiliates are either organized as limited partnerships, limited liability partnerships, limited liability companies, or corporations. AMG typically has contractual arrangements ("revenue sharing arrangements") with its Affiliates. In many cases, a percentage of revenue is allocable to fund Affiliate operating expenses, including compensation (the "Operating Allocation"), while the remaining revenue (the "Owners' Allocation") is allocable to AMG and the other partners or members. In other revenue sharing arrangements, AMG owns a minority interest that allocates to AMG a percentage of the Affiliate's revenue, with the remaining revenue available to the Affiliate to pay operating expenses and profit distributions. Under the revenue sharing arrangements, AMG's contractual share of revenue generally has priority over allocations to Affiliate management. In certain other cases, the Affiliate is not subject to a revenue sharing arrangement, but instead operates on a profit-based model. In these cases, AMG participates in any increases or decreases in the margin of such firms.
	(b)	Basis of Presentation
	The financial statements are prepared in accordance with accounting principles generally accepted in the U.S. ("GAAP"). All dollar amounts, except per share data in the text and tables herein, are stated in millions unless otherwise indicated. Certain reclassifications have been made to prior years' financial statements to conform to the current year's presentation.
	(c)	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts and disclosures in the financial statements. Actual results could differ from those estimates.
	(d)	Principles of Consolidation
	Investments in Affiliates
	The Company evaluates the risk, rewards, and significant terms of each of its Affiliate and other investments to determine the appropriate method of accounting. Majority-owned or otherwise controlled investments are consolidated. AMG typically has control over its Affiliate investments, directly or indirectly, as the sole general partner (in the case of Affiliates which are limited partnerships), the managing partner (in the case of Affiliates which are limited liability partnerships), the sole manager member (in the case of Affiliates which are limited liability companies) or principal shareholder (in the case of Affiliates which are corporations). As a result, the Company generally consolidates its Affiliate investments and all material intercompany balances and transactions are eliminated.
	For its consolidated Affiliates, the portion of the Owners’ Allocation allocated to Affiliate partners other than AMG is included in Net income (non-controlling interests) in the Consolidated Statements of Income. Non-controlling interests on the Consolidated Balance Sheets includes capital and undistributed Operating and Owners’ Allocation owned by Affiliate partners of the consolidated Affiliates. The effect of any changes in the Company's equity interests in its consolidated Affiliates resulting from the issuance or repurchase of an Affiliate's equity by the Company or one of its Affiliates is included as a component of stockholders' equity, net of the related income tax effect in the period of the change. The current redemption value of non-controlling interests has been presented as Redeemable non-controlling interests on the Consolidated Balance Sheets.
	AMG applies the equity method of accounting to investments where AMG does not hold a controlling equity interest but has the ability to exercise significant influence over operating and financial matters. In cases where AMG applies the equity method of accounting, it typically does not have an obligation to repurchase equity interests. Other investments in which AMG owns less than a 20% interest and does not exercise significant influence are accounted for under the cost method. Under the cost method, income is recognized as dividends when, and if, declared.
	Affiliate Sponsored Investment Vehicles
	The Company's Affiliates sponsor various investment vehicles where they also act as the investment advisor. Certain of these investment vehicles are variable interest entities (“VIEs”) while others are voting rights entities ("VREs").
	VIEs are consolidated if the Affiliate is determined to be the primary beneficiary (i.e. if it absorbs a majority of the expected losses, or receives a majority of the expected residual returns). In determining whether the Affiliate is the primary beneficiary, both qualitative and quantitative factors (e.g. the voting rights of the equity holders, economic participation of all parties, including how fees are earned and paid, related party ownership, guarantees and implied relationships) are considered.
	VREs are consolidated if the Affiliate is the managing member or general partner of the investment vehicle unless unaffiliated investors have certain rights to remove the Affiliate from such role or have substantive participating rights(e.g. approval of annual operating budgets, major financings, selection of senior management).
	(e)	Cash and Cash Equivalents
	The Company considers all highly liquid investments, including money market mutual funds, with original maturities of three months or less to be cash equivalents. Cash equivalents are stated at cost, which approximates market value due to the short-term maturity of these investments.
	(f)	Receivables
	The Company’s Affiliates earn advisory and performance fees which are billed monthly, quarterly or annually based on the terms of the related contracts. Billed but uncollected advisory and performance fees are presented within Receivables on the Consolidated Balance Sheets and are generally short-term in nature.
	Certain of the Company’s Affiliates in the United Kingdom act as an intermediary between clients and their sponsored funds. Normal settlement periods on transactions initiated by these clients result in unsettled fund share receivables and payables that are presented on a gross basis within Receivables and Payables and accrued liabilities on the Consolidated Balance Sheets. The gross presentation of these receivables and offsetting payables reflect the legal relationship between the underlying investor and the Company’s Affiliates.
	(g)	Investments in Marketable Securities
	Investments in marketable securities are classified as either trading or available-for-sale and carried at fair value. Unrealized gains or losses on investments classified as available-for-sale are reported, net of tax, as a separate component of Accumulated other comprehensive income in Equity until realized. Unrealized gains or losses related to trading securities are reported within Investment and other income in the period they occur. If a decline in the fair value of an available-for-sale investment is determined to be other than temporary, the carrying amount of the asset is reduced to its fair value, and the difference is charged to income in the period incurred.
	(h)	Fair Value Measurements
	The Company determines the fair value of certain investment securities and other financial and non-financial assets and liabilities. Fair value is determined based on the price that would be received for an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date, utilizing a hierarchy of three different valuation techniques:
	Level 1 - Unadjusted quoted market prices for identical instruments in active markets;
	Level 2 - Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations whose inputs, or significant value drivers, are observable; and
	Level 3 - Prices reflect the Company's own assumptions concerning unobservable inputs to the valuation model. These inputs require significant management judgment and reflect the Company's assumptions that market participants would use in pricing the asset or liability.
	(i)	Fixed Assets
	Fixed assets are recorded at cost and depreciated using the straight-line method over their estimated useful lives. The estimated useful lives of office equipment and furniture and fixtures range from three to ten years. Computer software developed or obtained for internal use is amortized using the straight-line method over the estimated useful life of the software, generally three to seven years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the term of the lease, and the building is amortized over 39 years. The costs of improvements that extend the life of a fixed asset are capitalized, while the cost of repairs and maintenance are expensed as incurred. Land is not depreciated.
	(i)	Leases
	The Company and its Affiliates currently lease office space and equipment under various leasing arrangements. As these leases expire, it can be expected that in the normal course of business they will be renewed or replaced. Leases are classified as either capital leases or operating leases, as appropriate. Most lease agreements classified as operating leases contain renewal options, rent escalation clauses or other inducements provided by the landlord. Rent expense is accrued to recognize lease escalation provisions and inducements provided by the landlord, if any, on a straight-line basis over the lease term.
	(j)	Equity Investments in Affiliates
	For equity method investments, the Company's portion of income or loss before taxes is included in Income from equity method investments, net of any amortization of intangible assets related to the Company's investment. The Company's share of income taxes incurred directly by Affiliates accounted for under the equity method are recorded within income taxes because these taxes generally represent the Company's share of the taxes incurred by the Affiliate.
	The Company periodically evaluates its equity method investments for impairment. In such impairment evaluations, the Company assesses if the fair value of the investment has declined below its carrying value for a period considered to be other than temporary. If the Company determines that a decline in fair value below the carrying value of the investment is other than temporary, then the reduction in carrying value would be recognized in Income from equity method investments in the Consolidated Statements of Income.
	(k)	Acquired Client Relationships and Goodwill
	Each acquired Affiliate has identifiable assets arising from contractual or other legal rights with their clients ("acquired client relationships"). In determining the value of acquired client relationships, the Company analyzes the net present value of each acquired Affiliate's existing client relationships based on a number of factors including: the Affiliate's historical and potential future operating performance; the Affiliate's historical and potential future rates of attrition among existing clients; the stability and longevity of existing client relationships; the Affiliate's recent, as well as long-term, investment performance; the characteristics of the firm's products and investment styles; the stability and depth of the Affiliate's management team and the Affiliate's history and perceived franchise or brand value.
	The Company has determined that certain of its mutual fund acquired client relationships meet the criteria to be considered indefinite-lived assets because the Company expects both the renewal of these contracts and the cash flows generated by these assets to continue indefinitely. Accordingly, the Company does not amortize these intangible assets, but instead reviews these assets annually or more frequently whenever events or circumstances occur indicating that the recorded indefinite-lived assets may be impaired. Each reporting period, the Company assesses whether events or circumstances have occurred which indicate that the indefinite life criteria are no longer met. If the indefinite life criteria are no longer met, the Company would assess whether the carrying value of the assets exceeds its fair value, an impairment loss would be recorded in an amount equal to any such excess and these assets would be reclassified to definite-lived.
	The expected useful lives of definite-lived acquired client relationships are analyzed each period and determined based on an analysis of the historical and projected attrition rates of each Affiliate's existing clients, and other factors that may influence the expected future economic benefit the Company will derive from the relationships.
	The Company tests for the possible impairment of indefinite and definite-lived intangible assets annually or more frequently whenever events or changes in circumstances indicate that the carrying amount of the asset is not recoverable. If such indicators exist, the Company compares the fair value of the asset to the carrying value of the asset. If the carrying value is greater than the fair value, an impairment loss may be recorded.
	Goodwill represents the future economic benefits arising from assets acquired in a business combination that are not individually identified and separately recognized, and is reported within the segments in which the Affiliate operates. Goodwill is not amortized, but is instead reviewed for impairment. The Company assesses goodwill for impairment at least annually, or more frequently whenever events or circumstances occur indicating that the recorded goodwill may be impaired. If the carrying amount of goodwill exceeds the fair value, an impairment loss may be recorded.
	(l)	Issuance Costs
	Issuance costs related to our senior bank debt are amortized over the remaining term of the credit facility and term loan. Costs incurred to issue debt are amortized over the shorter of the period to the first investor put or the Company's estimate of the expected term of the security. Costs associated with financial instruments that are not required to be accounted for separately as derivative instruments are charged directly to stockholders' equity.
	(m)	Derivative Financial Instruments
	The Company utilizes financial instruments, specifically interest rate derivative contracts to hedge certain interest rate exposures. In entering into these contracts, the Company intends to offset cash flow gains and losses that occur on its existing debt obligations with cash flow gains and losses on the contracts hedging these obligations.
	From time to time, the Company's Affiliates use foreign currency forward contracts to hedge the risk of currency exchange rate movements. In entering into these contracts, the Affiliates intend to offset cash flow gains and losses on projected foreign currency-denominated revenues and expenses as a result of variability in foreign currency exchange rates.
	The Company records all derivatives on the balance sheet at fair value. If the Company's derivatives qualify as cash flow hedges, the effective portion of the unrealized gain or loss is recorded in Accumulated other comprehensive income as a separate component of stockholders' equity and reclassified into earnings when the hedged cash flows are recorded in earnings. Hedge effectiveness is generally measured by comparing the present value of the cumulative change in the expected future variable cash flows of the hedged contract with the present value of the cumulative change in the expected future variable cash flows of the hedged item. To the extent that the critical terms of the hedged item and the derivative are not identical, hedge ineffectiveness would be reported in Investment and other income. If the Company's derivatives do not qualify as cash flow or fair value hedges, changes in the fair value of the derivatives are recognized as a gain or loss in Investment and other income.
	(n)	Contingent Payment Arrangements
	The Company periodically enters into contingent payment arrangements in connection with its business combinations. In these arrangements, the Company agrees to pay additional consideration to the sellers to the extent that certain levels of revenue growth are achieved. For consolidated Affiliates, the Company estimates the fair value of these potential future obligations at the time a business combination is consummated and records a liability on its Consolidated Balance Sheet. The Company then accretes the obligation to its expected payment amount over the period until the arrangement is measured. If the Company's expected payment amount subsequently changes, the obligation is reduced or increased in the current period resulting in a gain or loss, respectively. Both gains and losses resulting from changes to expected payments and the accretion of these obligations to their expected payment amounts are reflected within Imputed interest expense and contingent payment arrangements in the Company's Consolidated Statements of Income. For Affiliates accounted for under the equity method, the Company records a liability when a payment becomes probable with a corresponding increase to the carrying value of the Affiliate.
	(o)	Income Taxes
	The Company accounts for income taxes using the liability method. Under this method, deferred taxes are recognized for the expected future tax consequences of temporary differences between the book carrying amounts and tax bases of the Company's assets and liabilities. Deferred tax liabilities are generally attributable to intangible assets, convertible securities and deferred revenue. Deferred tax assets are generally attributable to state and foreign loss carryforwards, deferred compensation and the benefit of uncertain tax positions. Intangible-related deferred tax liabilities are not recorded on U.S. basis differences related to the Company's foreign investments in corporate stock because of the permanent nature of the Company's investments.
	In measuring the amount of deferred taxes each period, the Company must project the impact on its future tax payments of any reversal of deferred tax liabilities (which would increase the Company's tax payments), and any use of its state and foreign loss carryforwards (which would decrease its tax payments). In forming these estimates, the Company uses enacted federal, state and foreign income tax rates and makes assumptions about the apportionment of future taxable income to jurisdictions in which the Company has operations. An increase or decrease in foreign, federal or state income tax rates could have a material impact on the Company's deferred income tax liabilities and assets and would result in a current income tax charge or benefit.
	The Company recognizes the financial statement benefit of an uncertain tax position only after considering the probability that a tax authority would sustain the position in an examination. For tax positions meeting a "more-likely-than-not" threshold, the amount recognized in the financial statements is the benefit expected to be realized upon settlement with the tax authority. For tax positions not meeting the threshold, no financial statement benefit is recognized. The Company recognizes interest and other charges relating to unrecognized tax benefits as additional tax expense.
	In the case of the Company's deferred tax assets, the Company regularly assesses the need for valuation allowances, which would reduce these assets to their recoverable amounts. In forming these estimates, the Company makes assumptions of future taxable income that may be generated to utilize these assets, which have limited lives. If the Company determines that these assets will be realized, the Company records an adjustment to the valuation allowance, which would decrease tax expense in the period such determination was made. Likewise, should the Company determine that it would be unable to realize additional amounts of deferred tax assets, an adjustment to the valuation allowance would be charged to tax expense in the period such determination was made.
	(p)	Foreign Currency Translation
	The assets and liabilities of Affiliates whose functional currency is not the U.S. dollar are translated into U.S. dollars using exchange rates in effect as of the balance sheet date. The revenue and expenses of these Affiliates are translated into U.S. dollars using average exchange rates for the relevant period. Because of the permanent nature of the Company's investments, net translation exchange gains and losses are excluded from Net income but are recorded in Other comprehensive income. Foreign currency transaction gains and losses are reflected in Investment and other income.
	(q)	Concentration of Credit Risk
	Financial instruments that potentially subject the Company to significant concentrations of credit risk consist principally of cash investments. The Company maintains cash and cash equivalents, investments and, at times, certain financial instruments with various high credit-quality financial institutions. These financial institutions are typically located in countries in which AMG and its Affiliates operate. For AMG and certain Affiliates, cash deposits at a financial institution may exceed Federal Deposit Insurance Corporation insurance limits.
	(r)	Revenue Recognition
	The Company's consolidated revenue primarily represents advisory fees billed by Affiliates for managing the assets of clients. Asset-based advisory fees are recognized as services are rendered and are based upon a percentage of the value of client assets managed. Any fees collected in advance are deferred and recognized as income over the period earned. Performance-based advisory fees are generally assessed as a percentage of the investment performance realized on a client's account, generally over an annual period. Performance-based advisory fees are recognized when they are earned (i.e. when they become billable to customers and are not subject to contingent repayment) based on the contractual terms of agreements and when collection is reasonably assured. Carried interest is recognized upon the earlier of the termination of the investment product or when the likelihood of claw-back is improbable. Also included in revenue are commissions earned by broker dealers, recorded on a trade date basis, and other service fees recorded as earned.
	The Company's Affiliates have contractual arrangements with third parties to provide certain distribution-related services. These third parties are primarily compensated based on the value of client assets over time. Distribution related revenues are presented gross of any related expenses when the Affiliate is the principal in its role as primary obligor under its sales and distribution arrangements. Distribution-related expenses are presented within Selling, general and administrative expenses.
	(s)	Earnings Per Share
	The calculation of basic earnings per share is based on the weighted average number of shares of the Company's common stock outstanding during the period. Diluted earnings per share is similar to basic earnings per share, but adjusts for the dilutive effect of the potential issuance of incremental shares of the Company's common stock.
The Company had convertible securities outstanding during the periods presented and is required to apply the if-converted method to these securities in its calculation of diluted earnings per share. Under the if-converted method, shares that are issuable upon conversion are deemed outstanding, regardless of whether the securities are contractually convertible into the Company's common stock at that time. For this calculation, the interest expense (net of tax) attributable to these dilutive securities is added back to Net income (controlling interest), reflecting the assumption that the securities have been converted. Issuable shares for these securities and related interest expense are excluded from the calculation if an assumed conversion would be anti-dilutive to diluted earnings per share.
(t)	Share-Based Compensation Plans
The Company recognizes expenses for all share-based payments based on their grant date fair values over the requisite service period.
The Company reports any tax benefits realized upon the exercise of stock options that are in excess of the expense recognized for reporting purposes as a financing activity in the Company's Consolidated Statements of Cash Flows. If the tax benefit realized is less than the expense, the tax shortfall is recognized in stockholders' equity. To the extent the expense exceeds available windfall tax benefits, it is recognized in the Consolidated Statements of Income. The Company was permitted to calculate its cumulative windfall tax benefits for the purposes of accounting for future tax shortfalls. The Company elected to apply the long-form method for determining the pool of windfall tax benefits.
(u)	Segment Information
Management has assessed and determined that the Company operates in three business segments representing the Company's three principal distribution channels: Institutional, Mutual Fund and High Net Worth, each of which has different client relationships.
Revenue in the Institutional distribution channel is earned from relationships with public and private client entities, including pension plans, foundations, endowments and sovereign wealth funds. Revenue in the Mutual Fund distribution channel is earned from advisory and sub-advisory relationships with all domestically-registered investment products as well as investment products that are registered abroad. Revenue in the High Net Worth distribution channel is earned from relationships with ultra-high and high net worth individuals, family trusts and managed account programs.
Revenue earned from client relationships managed by Affiliates accounted for under the equity method is not consolidated with the Company's reported Revenue but instead is included (net of operating expenses, including amortization) in Income from equity method investments, and reported in the distribution channel in which the Affiliate operates.
In reporting segment operating expenses, consolidated Affiliate expenses are allocated to a particular segment on a pro rata basis with respect to the revenue generated by that Affiliate in such segment. Generally, as revenue increases, additional compensation is typically paid to Affiliate management partners from the Operating Allocation. As a result, the contractual expense allocation pursuant to a revenue sharing arrangement may result in the characterization of any growth in margin beyond the Company's Owners' Allocation as an operating expense. Unless specific to a distribution channel, all other operating and non-operating expenses have been allocated to segments based on the proportion of cash flow distributions reported by Affiliates in each segment.
(v)	Recent Accounting Developments
In April 2014, the Financial Accounting Standards Board (the "FASB") issued an update to the guidance for discontinued operations accounting and reporting. The new guidance amends the definition of discontinued operations and requires entities to provide additional disclosures regarding disposal transactions that do not meet the discontinued operations criteria. The new guidance is effective for interim and fiscal periods beginning after December 15, 2014. The Company does not expect this guidance to have a significant impact on the Consolidated Financial Statements.
In May 2014, the FASB issued a final standard on revenue from contracts with customers. The new standard provides a comprehensive model for revenue recognition. The new standard is effective for interim and fiscal periods beginning after December 15, 2016. The Company is evaluating the impact of this standard on its Consolidated Financial Statements.
In November 2014, the FASB issued an update to the guidance for hybrid financial instruments. The new guidance requires entities to determine the nature of the host contract by considering all stated and implied substantive terms and features of a hybrid financial instrument, weighing each term and feature on the basis of relevant facts and circumstances. The new guidance is effective for interim and fiscal periods beginning after December 15, 2015. The Company does do not expect this guidance to have a significant impact on its Consolidated Financial Statements.
In February 2015, the FASB issued a new standard that amended the current consolidation guidance. The new standard changes the analysis required to determine whether an entity is a variable interest entity and should be consolidated. The new standard is effective for interim and fiscal periods beginning after December 15, 2015, and early adoption is permitted. The Company is evaluating the impact of this new standard on its Consolidated Financial Statements.

Investments_in_Marketable_Secu

Investments in Marketable Securities	12 Months Ended
	Dec. 31, 2014
Marketable Securities [Abstract]
Investments in Marketable Securities	Investments in Marketable Securities
	Investments in marketable securities at December 31, 2013 and 2014 were $157.9 million and $172.6 million, respectively. The following is a summary of the cost, gross unrealized gains and losses and fair value of investments classified as available-for-sale and trading at December 31, 2013 and 2014:
		Available-for-Sale						Trading
		31-Dec-13			31-Dec-14			31-Dec-13			December 31, 2014
	Cost	$	103.2		$	125.6		$	17.9		$	19.5
	Unrealized Gains	33.3			42.8			4.6			2.9
	Unrealized Losses	(1.1		)	(18.1		)	(0.0		)	(0.1		)
	Fair Value	$	135.4		$	150.3		$	22.5		$	22.3
	There were no significant realized gains or losses on investments classified as available-for-sale for the year ended December 31, 2013. For the year ended December 31, 2014, there were $3.4 million of realized gains on investments classified as available-for-sale.

Other_Investments

Other Investments	12 Months Ended
	Dec. 31, 2014
Investments, All Other Investments [Abstract]
Other Investments	Other Investments
	Other investments consist of investments in funds advised by Affiliates which are carried at fair value. The income or loss related to these investments is classified within Investment and other income in the Consolidated Statements of Income.

Variable_Interest_Entities

Variable Interest Entities	12 Months Ended
	Dec. 31, 2014
Variable Interest Entities
Variable Interest Entities	Variable Interest Entities
	The Company's Affiliates act as the investment manager for certain VIEs. Affiliates are entitled to receive management fees and may be eligible, under certain circumstances, to receive performance fees. The Affiliates' exposure to risk in these entities is generally limited to any equity investment and any uncollected management or performance fees, neither of which were material at December 31, 2013 and 2014. The Affiliates do not have any investment performance guarantees to these VIEs.
	The Affiliates are not the primary beneficiary of any of these VIEs as their involvement is limited to that of a service provider and their investment, if any, represents an insignificant interest in the fund's assets under management. Since the Affiliates' variable interests will not absorb the majority of the variability of the entity's net assets, these entities are not consolidated.
	The net assets and liabilities of these unconsolidated VIEs and the Company's maximum risk of loss are as follows:
		31-Dec-13				31-Dec-14
	Category of Investment	Unconsolidated		Carrying Value and		Unconsolidated		Carrying Value and
		VIE Net Assets		Maximum Exposure		VIE Net Assets		Maximum Exposure
				to Loss				to Loss
	Sponsored investment funds	$	8,112.70	$	1.7	$	8,550.40	$	1.2

Senior_Bank_Debt

Senior Bank Debt	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Senior Bank Debt	Senior Bank Debt
	The Company has a $1.25 billion senior unsecured revolving credit facility (the "credit facility") which matures in April 2018. The Company pays interest on any outstanding obligations at specified rates, based either on the LIBOR rate or the prime rate as in effect from time to time.
	As of December 31, 2013 and 2014, the Company had outstanding borrowings under the credit facility of $525.0 million and $605.0 million, respectively, and the weighted-average interest rate on outstanding borrowings was 1.95% and 1.40%, respectively. The Company pays commitment fees on the unused portion of its credit facility. In 2013 and 2014 these fees amounted to $3.2 million and $2.8 million, respectively.
	On April 15, 2014, the Company entered into a $250.0 million five-year senior unsecured term loan, which matures in 2019. The Company pays interest at specified rates (based on either the LIBOR rate or the prime rate as in effect from time to time). Subject to certain conditions, the Company may borrow up to an additional $100.0 million. As of December 31, 2014, the Company had outstanding borrowings under the term loan of $250.0 million, and the weighted-average interest rate on outstanding borrowings was 1.48%.
	The credit facility and term loan contain financial covenants with respect to leverage and interest coverage, as well as customary affirmative and negative covenants, including limitations on priority indebtedness, liens, cash dividends, asset dispositions, fundamental corporate changes and certain customary events of default.

Senior_Notes

Senior Notes	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Senior Notes	Senior Notes
	At December 31, 2014, the Company had three senior notes outstanding. The principal terms of these notes are summarized below.
		2022			2024			2042
		Senior			Senior			Senior
		Notes			Notes			Notes
	Issue date	Oct-12			Feb-14			Aug-12
	Maturity date	Oct-22			Feb-24			Aug-42
	Potential Call Date	Oct-15			Any Time (1)			Aug-17
	Par value (in millions)	$	140		$	400		$	200
	Call Price	At Par			As Defined (1)			At Par
	Stated coupon	5.25		%	4.25		%	6.375		%
	Coupon frequency	Quarterly			Semi-annually			Quarterly
	_______________________________________________________________________________
	-1	The 2024 senior notes may be redeemed at any time, in whole or in part, at a make-whole redemption price plus accrued and unpaid interest.
	On February 13, 2015, the Company issued $350.0 million aggregate principal amount of 3.50% senior notes due 2025 (the "2025 senior notes"). The unsecured 2025 senior notes pay interest semi-annually and may be redeemed at any time, in whole or in part, at a make-whole redemption price plus accrued and unpaid interest. In addition to customary event of default provisions, the indenture limits the Company's ability to consolidate, merge or sell all or substantially all of its assets.

Convertible_Securities

Convertible Securities	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Convertible Securities	Convertible Securities
	In 2013, the Company settled $460.0 million principal amount outstanding of our 3.95% senior convertible securities due 2038 (“2008 senior convertible securities”). In connection with the settlement, the Company paid an aggregate of $641.3 million and all of our 2008 senior convertible notes have been retired.
	On February 13, 2014, the Company delivered a notice to redeem all $300.0 million principal amount of its outstanding junior convertible trust preferred securities issued in 2006 ("2006 junior convertible securities"). In lieu of redemption, substantially all holders of the 2006 junior convertible securities elected to convert their securities into a defined number of shares. The Company issued 1.9 million shares of its common stock and recognized a loss of $18.8 million, which is included in Imputed interest expense and contingent payment arrangements. All of the Company's 2006 junior convertible securities have been canceled and retired.
	At December 31, 2014, the Company had junior convertible trust preferred securities outstanding that were issued in 2007 ("2007 junior convertible securities").
	The carrying values of the Company's convertible securities are as follows:
		31-Dec-13				31-Dec-14
		Carrying		Principal amount		Carrying		Principal amount
		Value		at maturity		Value		at maturity
	Junior convertible securities:
	2007 junior convertible securities(1)	$	301.2	$	430.8	$	303.1	$	430.8
	2006 junior convertible securities	217.5		300		—		—
	Total junior convertible securities	$	518.7	$	730.8	$	303.1	$	430.8
	_______________________________________________________________________________
	-1	The carrying value is accreted to the principal amount at maturity over a remaining life of 23 years.
	The 2007 junior convertible securities bear interest at 5.15% per annum, payable quarterly in cash. Each $50 security is convertible, at any time, into 0.25 shares of the Company's common stock, which represents a conversion price of $200 per share. Holders of the securities have no rights to put these securities to the Company. Upon conversion, holders will receive cash or shares of the Company's common stock, or a combination therefore, at the Company's election. The Company may redeem the 2007 junior convertible securities if the closing price of its common stock exceeds $260 per share for 20 trading days in a period of 30 consecutive trading days. These convertible securities are considered contingent payment debt instruments under federal income tax regulations, which require the Company to deduct interest in an amount greater than its reported interest expense. These deductions result in annual deferred tax liabilities of approximately $8.7 million. These deferred tax liabilities will be reclassified directly to stockholders' equity if the Company's common stock is trading above certain thresholds at the time of the conversion of the securities.

Forward_Equity_Sale_Agreement

Forward Equity Sale Agreement	12 Months Ended
	Dec. 31, 2014
Forward Equity Sale Agreements
Forward Equity Sale Agreement	Forward Equity Sale Agreement
	Under a forward equity agreement, the Company may sell shares of common stock up to an aggregate notional amount of $400.0 million. During 2012, the Company entered into contracts to sell a notional amount of $147.2 million at an average share price of $121.37. During 2013, the Company agreed to net settle $77.0 million notional amount of forward equity contracts for cash at an average share price of $185.56. During 2014, the Company net settled $70.2 million notional amount of forward equity contracts at an average share price of $198.71. At December 31, 2014, the Company had $252.8 million remaining notional amount that were available to sell under the forward equity agreement.

Derivative_Financial_Instrumen

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments	Derivative Financial Instruments
	From time to time, the Company seeks to offset its exposure to changing interest rates and its Affiliates seek to offset exposure to foreign currency exchange rates by entering into interest rate hedging contracts and foreign currency forward contracts, respectively.
	In 2010, the Company entered into interest rate swap agreements as summarized in the table below:
		Notional			Paying			Receiving		Start Date		Expiration Date
		Amount
	Counterparty A	$	25		1.67	%		3-Month LIBOR		Oct-10		Oct-15
	Counterparty A	$	25		1.65	%		3-Month LIBOR		Oct-10		Oct-15
	Counterparty B	$	25		1.59	%		3-Month LIBOR		Oct-10		Oct-15
	Counterparty B	$	25		2.14	%		3-Month LIBOR		Oct-10		Oct-17
	The Company’s interest rate derivative contracts contain provisions that may require the Company or the counterparties to post collateral based upon the current fair value of the derivative contracts. As of December 31, 2014, the Company had posted collateral of $2.4 million related to its interest rate swap contracts.
	In 2014, one of the Company's Affiliates entered into foreign currency forward contracts relating to projected expenses denominated in pound sterling (GBP) and projected revenues denominated in euro (EUR):
			Paying			Receiving				Start Date		Expected Settlement
	Counterparty C		$	21.9		£	13.5			Sep-14		Monthly 2015
	Counterparty C		€	28		$	34.3			Dec-14		Quarterly 2015
	The following summarizes the amount of derivative instrument gains and losses reported in the Consolidated Statements of Comprehensive Income:
		For the Years Ended December 31,
	Cash Flow Hedges	2012			2013				2014
	Interest rate swaps	$	(1.1	)	$	1.5			$	1.1
	Treasury rate locks	0			0				0
	Foreign currency forward contracts	—			—				(0.5		)
	Total	$	(1.1	)	$	1.5			$	0.6
	At December 31, 2013 and 2014, the fair values of the Company's interest rate swaps ($2.5 million and $1.4 million, respectively) are presented within Other liabilities and its foreign currency forward contracts ($0.5 million at December 31, 2014) are presented within Other assets.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies
	The Company and its Affiliates are subject to claims, legal proceedings and other contingencies in the ordinary course of their business activities. Each of these matters is subject to various uncertainties, and it is possible that some of these matters may be resolved in a manner unfavorable to the Company or its Affiliates. The Company and its Affiliates establish accruals for matters for which the outcome is probable and the amount of the liability can be reasonably estimated. Management believes that any liability in excess of these accruals upon the ultimate resolution of these matters will not have a material adverse effect on the Company.
	Certain Affiliates operate under regulatory authorities which require that they maintain minimum financial or capital requirements. Managements is not aware of any significant violations of such financial requirements occurring during the period.
	The Company has committed to co-invest in certain investment partnerships. As of December 31, 2014, these unfunded commitments were $67.8 million and may be called in future periods. In connection with a past acquisition agreement, the Company is contractually entitled to reimbursement from a prior owner for $21.1 million of these commitments if they are called.
	Under past acquisition agreements, the Company is contingently liable, upon achievement by Affiliates of specified financial targets, to make payments of up to $276.0 million through 2017. As of December 31, 2014, the Company expects to make payments of $75.0 million ($17.5 million in 2015) to settle obligations related to consolidated Affiliates and may make payments of up to $201.0 million related to the Company's equity method investments. The net present value of the expected payments for consolidated Affiliates totals $59.3 million as of December 31, 2014.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements
	The following table summarizes the Company's financial assets and liabilities that are measured at fair value on a recurring basis:
						Fair Value Measurements
		31-Dec-13
						Level 1				Level 2				Level 3
	Financial Assets
	Cash equivalents	$	39			$	39			$	—			$	—
	Investments in marketable securities(1)
	Trading securities	22.5				22.5				—				—
	Available-for-sale securities	135.4				135.4				—				—
	Other investments	164.3				14.1				18.4				131.8
	Financial Liabilities(2)
	Contingent payment arrangements	$	50.2			$	—			$	—			$	50.2
	Obligations to related parties	76.9				—				—				76.9
	Interest rate swaps	2.5				—				2.5				—
						Fair Value Measurements
		31-Dec-14
						Level 1				Level 2				Level 3
	Financial Assets
	Cash equivalents	$	59.1			$	59.1			$	—			$	—
	Investments in marketable securities(1)
	Trading securities	22.3				22.3				—				—
	Available for sale securities	150.3				150.3				—				—
	Other investments	167.2				13.6				19.4				134.2
	Financial Liabilities(2)
	Contingent payment arrangements	$	59.3			$	—			$	—			$	59.3
	Obligations to related parties	93.1				—				—				93.1
	Interest rate swaps	1.4				—				1.4				—
	Foreign currency forward contracts	0.5				—				0.5				—
	_______________________________________________________________________________
	-1	Principally investments in equity securities.
	-2	Amounts are presented within Other liabilities in the accompanying Consolidated Balance Sheets.
	The following is a description of the significant financial assets and liabilities measured at fair value and the fair value methodologies used.
	Cash equivalents consist primarily of highly liquid investments in daily redeeming money market funds which are classified as Level 1.
	Investments in marketable securities consist primarily of investments in publicly traded securities and in funds advised by Affiliates which are valued using net asset value ("NAV"). Publicly traded securities and investments in daily redeeming funds that calculate NAVs are classified as Level 1.
	Other investments consist primarily of funds advised by Affiliates and are valued using NAV. Investments in daily redeeming funds that calculate NAVs are classified as Level 1. Investments in funds that permit redemptions monthly or quarterly are classified as Level 2. Investments in funds that are subject to longer redemption restrictions are classified as Level 3. The fair value of Level 3 assets is determined using NAV one quarter in arrears (adjusted for current period calls and distributions).
	Contingent payment arrangements represent the present value of the expected future settlement of contingent payment arrangements related to the Company's investments in consolidated Affiliates. The significant unobservable inputs that are used in the fair value measurement of these obligations are growth and discount rates. Increases in the growth rate result in a higher obligation while an increase in the discount rate results in a lower obligation.
	Obligations to related parties include agreements to repurchase Affiliate equity and liabilities offsetting certain investments which are held by the Company but economically attributable to a related party. The significant unobservable inputs that are used in the fair value measurement of the agreements to repurchase Affiliate equity are growth and discount rates. Increases in the growth rate result in a higher obligation while an increase in the discount rate results in a lower obligation. The liability to a related party is measured based upon certain investments held by the Company, the fair value of which is determined using NAV one quarter in arrears adjusted for current period calls and distributions.
	Interest rate swaps and foreign currency forward contracts use model-derived valuations in which all significant inputs are observable in active markets to determine the fair value of these derivatives.
	It is the Company's policy to value financial assets or liabilities transferred as of the beginning of the period in which the transfer occurs. There were no significant transfers of financial assets or liabilities from Level 1 to Level 2 in 2014 or 2013.
	Level 3 Financial Assets and Liabilities
	The following table presents the changes in Level 3 assets and liabilities for the years ended December 31, 2013 and 2014:
		For the Years Ended December 31,
		2013												2014
		Other Investments				Contingent Payment Arrangements				Obligations to Related Parties				Other Investments				Contingent Payment Arrangements			Obligations to Related Parties
	Balance, beginning of period	$	118.9			$	31			$	77.8			$	131.8			$	50.2		$	76.9
	Net gains/losses	12.3			(1)	19.2			(2)	6.2			(3)	11.4			(1)	9.1		(2)	5.5			(3)
	Purchases and issuances	18				—				62.9				17.4				—			96.7
	Settlements and reductions	(17.4		)		—				(70.0		)		(26.4		)		—			(86.0		)
	Net transfers in and/or out of Level 3	—				—				—				—				—			—
	Balance, end of period	$	131.8			$	50.2			$	76.9			$	134.2			$	59.3		$	93.1
	Net unrealized gains/losses relating to instruments still held at the reporting date	$	16.5			$	19.2			$	2.2			$	17.1			$	9.1		$	(0.1	)
	_______________________________________________________________________________
	-1	Gains and losses on Other investments are recorded in Investment and other income.
	-2	Accretion and changes to payment estimates under the Company's contingent payment arrangements are recorded in Imputed interest expense and contingent payment arrangements.
	-3	Gains and losses associated with agreements to repurchase Affiliate equity are recorded in Imputed interest expense and contingent payment arrangements. Gains and losses related to liabilities offsetting certain investments are recorded in Investment and other income.
	The following table presents certain quantitative information about the significant unobservable inputs used in valuing the Company's Level 3 financial liabilities:
			Quantitative Information about Level 3 Fair Value Measurements
			Valuation		Unobservable Input		Fair Value at				Range at December 31, 2013		Fair Value at				Range at December 31, 2014
			Techniques				December 31, 2013						December 31, 2014
	Contingent payment arrangements		Discounted cash flow		Growth rates		$	50.2			3% - 11%		$	59.3			6%
					Discount rates						14% - 18%						15%
	Affiliate equity repurchase obligations		Discounted cash flow		Growth rates		4				8%		21.5				5% - 9%
					Discount rates						15%						15% - 16%
Investments in Certain Entities that Calculate Net Asset Value
The Company uses the NAV of certain investments as their fair value. The NAVs that have been provided by the investees have been derived from the fair values of the underlying investments as of the measurement dates. The following table summarizes, as of December 31, 2013 and 2014, the nature of these investments and any related liquidity restrictions or other factors which may impact the ultimate value realized:
	31-Dec-13								31-Dec-14
Category of Investment	Fair Value				Unfunded				Fair Value				Unfunded
					Commitments								Commitments
Private equity(1)	$	131.8			$	62.9			$	134.2			$	67.8
Other funds(2)	82.3				—				75.8				—
	$	214.1			$	62.9			$	210			$	67.8
_______________________________________________________________________________
-1	These funds primarily invest in a broad range of private equity funds, as well as make direct investments. Distributions will be received as the underlying assets are liquidated over the life of the funds.
-2	These are multi-disciplinary funds that invest across various asset classes and strategies including long/short equity, credit and real estate. Investments are generally redeemable on a daily or quarterly basis.
There are no current plans to sell any of these investments.
Other Financial Assets and Liabilities Not Carried at Fair Value
The carrying amount of cash, cash equivalents, receivables, and payables and accrued liabilities approximates fair value because of the short-term nature of these instruments. The carrying value of notes receivable approximates fair value because interest rates and other terms are at market rates. The carrying value of senior bank debt approximates fair value because the debt has variable interest based on selected short-term rates. The following table summarizes the Company's other financial liabilities not carried at fair value:
	31-Dec-13								31-Dec-14
	Carrying Amount				Fair Value				Carrying Amount				Fair Value				Fair Value Hierarchy
Senior notes	$	340			$	325			$	736.8			$	786.2			Level 2
Convertible securities	518.7				963.9				303.1				532.1				Level 2

Business_Combinations

Business Combinations	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Business Combinations	Business Combinations
	The Company completed majority investments in three new Affiliates. SouthernSun Asset Management, LLC ("SouthernSun") was completed on March 31, 2014, River Road Asset Management, LLC ("River Road") was completed on June 30, 2014 and Veritas Asset Management, LLP ("Veritas") was completed on October 31, 2014. The Company completed its purchase price allocations for these new Affiliates by using a financial model that includes assumptions of market performance, net client cash flows and discount rates. There were no significant differences between the provisional and final purchase price allocations. The consideration paid (less net tangible assets acquired) is deductible for U.S. tax purposes over a 15-year life.
	The purchase price allocation for these investments is as follows:
		SouthernSun			River Road				Veritas
	Consideration paid	$	109.9		$	132.4			$	319.9
	Non-controlling interests	74			35.8				125.2
	Enterprise value	$	183.9		$	168.2			$	445.1
	Acquired client relationships	$	125.3		$	64.1			$	275.6
	Deferred income taxes	—			—				(35.7		)
	Tangible assets, net	6.3			4.5				12.6
	Goodwill	52.3			99.6				192.6
		$	183.9		$	168.2			$	445.1
	The excess of the enterprise value over the net assets acquired was recorded as goodwill and the segment allocation is as follows:
		SouthernSun		River Road			Veritas
	Institutional	37	%	31	%		24	%
	Mutual Fund	23	%	51	%		76	%
	High Net Worth	40	%	18	%		—	%
	Unaudited pro-forma financial results are set forth below assuming these investments occurred on January 1, 2013, the revenue sharing arrangement had been in effect for the entire period and after making certain pro-forma adjustments.
		For the Years Ended December 31,
		2013 (Unaudited)			2014 (Unaudited)
	Revenue	$	2,353.50		$	2,644.90
	Net income (controlling interest)	382.3			474.9
	Earnings per share (basic)	$	7.2		$	8.63
	Earnings per share (diluted)	$	6.94		$	8.4
	The unaudited pro-forma financial results are not necessarily indicative of the financial results had the investments been consummated at the beginning of the periods presented, nor are they necessarily indicative of the financial results expected in future periods. The unaudited pro-forma financial results do not include the impact of transaction and integration related costs or benefits that may be expected to result from these investments.
	The Company's investments in SouthernSun, River Road and Veritas contributed $72.2 million and $8.5 million to the Company's revenue and earnings, respectively, during 2014.

Goodwill_and_Acquired_Client_R

Goodwill and Acquired Client Relationships	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Acquired Client Relationships	Goodwill and Acquired Client Relationships
	The following tables presents the change in goodwill and components of intangible assets of the Company's Affiliates that are consolidated during 2013 and 2014:
		Goodwill
		Institutional			Mutual Fund			High Net Worth			Total
	Balance, as of December 31, 2012	$	1,078.50		$	939.5		$	337.2		$	2,355.20
	Goodwill acquired	—			—			—			—
	Foreign currency translation	(2.2		)	(11.4		)	0.1			(13.5		)
	Balance, as of December 31, 2013	$	1,076.30		$	928.1		$	337.3		$	2,341.70
	Goodwill acquired	97.2			208.1			39.2			344.5
	Foreign currency translation	(14.4		)	(10.9		)	(8.1		)	(33.4		)
	Balance, as of December 31, 2014	$	1,159.10		$	1,125.30		$	368.4		$	2,652.80
		Acquired Client Relationships
		Definite-lived									Indefinite-lived			Total
		Gross Book			Accumulated			Net Book			Net Book			Net Book
		Value			Amortization			Value			Value			Value
	Balance, as of December 31, 2012	$	1,109.60		$	(383.5	)	$	726.1		$	859.4		$	1,585.50
	New Investments	—			—			—			—			—
	Amortization and impairments	—			(128.2		)	(128.2		)	—			(128.2		)
	Foreign currency translation	(1.2		)	—			(1.2		)	4.6			3.4
	Transfers and other	(68.9		)	68.9			—			—			—
	Balance, as of December 31, 2013	$	1,039.50		$	(442.8	)	$	596.7		$	864		$	1,460.70
	New Investments	220.3			—			220.3			244.7			465
	Amortization and impairments	—			(122.2		)	(122.2		)	—			(122.2		)
	Foreign currency translation	(4.7		)	—			(4.7		)	(20.4		)	(25.1		)
	Transfers and other	—			—			—			—			—
	Balance, as of December 31, 2014	$	1,255.10		$	(565.0	)	$	690.1		$	1,088.30		$	1,778.40
	For the Company's Affiliates that are consolidated, definite-lived acquired client relationships are amortized over their expected useful lives. As of December 31, 2014, these relationships were being amortized over a weighted average life of approximately ten years. The Company estimates that its consolidated annual amortization expense will be approximately $120 million for each of the next five years, assuming no additional investments in new or existing Affiliates.
	During 2013 and 2014, the Company completed impairment assessments on its goodwill and definite-lived and indefinite-lived acquired client relationships and no impairments were indicated.

Equity_Method_Investments_in_A

Equity Method Investments in Affiliates	12 Months Ended
	Dec. 31, 2014
Equity Method Investments and Joint Ventures [Abstract]
Equity Investments in Affiliates	Equity Method Investments in Affiliates
	On April 1, 2014, the Company completed its investment in EIG Global Energy Partners, LLC ("EIG"). The Company completed its purchase price allocation using a financial model that included assumptions of market performance, net client flows and discount rates and resulted in $215.2 million being allocated to definite-lived acquired client relationships. The consideration paid (less net tangible assets acquired) is deductible for U.S. tax purposes over a 15-year life.
	On December 26, 2014, the Company completed an additional investment in AQR Capital Management Holdings, LLC. The Company's provisional purchase price allocation was measured using a financial model that includes assumptions of expected market performance, net client flows and discount rates and resulted in $122.2 million and $90.0 million being allocated to definite-lived and indefinite-lived acquired client relationships, respectively. This provisional amount may be revised upon completion of the final valuation. The consideration paid (less net tangible assets acquired) will be deductible for U.S. tax purposes over a 15-year life.
	The intangible assets at the Company's equity method Affiliates consist of definite-lived and indefinite-lived acquired client relationships and goodwill. As of December 31, 2014, the definite-lived relationships were being amortized over a weighted average life of approximately fourteen years. The Company recognized amortization expense for these relationships of $41.7 million for the year ended December 31, 2013, as compared to $32.3 million for the year ended December 31, 2014. Assuming no additional investments in new or existing Affiliates, the Company estimates the annual amortization expense attributable to its current equity-method Affiliates to be approximately $32 million for each of the next five years.
	The following table presents summarized financial information for Affiliates accounted for under the equity method.
		For the Years Ended December 31,
		2012		2013		2014
	Revenue(1)	$	1,005.40	$	1,589.60	$	1,869.30
	Net income	783.3		1,321.90		253.8
		December 31,
		2013		2014
	Assets(2)	$	19,139.70	$	34,729.20
	Liabilities and Non-controlling interest(2)	17,763.50		33,048.60
	_______________________________________________________________________________
	-1	Revenue includes advisory fees for asset management services and net investment income from consolidated investment partnerships.
	-2	Assets consist primarily of investment securities in consolidated investment partnerships. These partnerships are generally held by investors that are unrelated to the Company.
	The Company's share of undistributed earnings from equity method investments totaled $133.9 million as of December 31, 2014.

Fixed_Assets_and_Lease_Commitm

Fixed Assets and Lease Commitments	12 Months Ended
	Dec. 31, 2014
Fixed Assets and Lease Commitments [Abstract]
Fixed Assets and Lease Commitments	Fixed Assets and Lease Commitments
	Fixed assets consisted of the following:
		December 31,
		2013			2014
	Building and leasehold improvements	$	78.5		$	84.1
	Equipment	36.5			38.3
	Furniture and fixtures	15.4			17.5
	Land and improvements	18.9			18.7
	Software	30.6			40.8
	Fixed assets, at cost	179.9			199.4
	Accumulated depreciation and amortization	(87.6		)	(104.0		)
	Fixed assets, net	$	92.3		$	95.4
	The Company and its Affiliates lease office space and equipment for their operations. At December 31, 2014, the Company's aggregate future minimum payments for operating leases having initial or non-cancelable lease terms greater than one year are payable as follows:
		Required Minimum
		Payments
	2015	$	33.8
	2016	28.8
	2017	25.2
	2018	22.8
	2019	21.6
	Thereafter	69.4
	Consolidated rent expense for 2012, 2013 and 2014 was $30.5 million, $30.3 million and $30.5 million, respectively.

Accounts_Payable_and_Accrued_L

Accounts Payable and Accrued Liabilities	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Accounts Payable and Accrued Liabilities	Payables and Accrued Liabilities
	Payables and accrued liabilities consisted of the following:
		December 31,
		2013		2014
	Accrued compensation and distributions	$	211.4	$	491.3
	Unsettled fund share payables	84.6		78.3
	Accrued income taxes	73		59.8
	Accrued share repurchases	—		47.8
	Accrued professional fees	33.4		26.9
	Other	112.3		104.2
		$	514.7	$	808.3

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions
	The Company's Affiliates generally pay quarterly distributions to Affiliate management owners. Distributions paid to Affiliate management owners were $181.4 million, $267.1 million and $569.4 million for the years ended December 31, 2012, 2013 and 2014, respectively.
	The Company periodically issues Affiliate equity interests to and repurchases Affiliate equity interests from its Affiliate partners.  The amount paid for repurchases was $8.0 million, $36.9 million, and $32.6 million for the years ended December 31, 2012, 2013 and 2014, respectively.  The total amount received for issuances was $3.3 million, $2.5 million, and $11.0 million for the years ended December 31, 2012, 2013 and 2014, respectively.
	The Company may record amounts receivable and payable to Affiliate partners in connection with the transfer of Affiliate equity interests that have not settled at the end of the period. The Company also has liabilities to related parties for deferred purchase price and contingent payment arrangements in connection with certain business combinations. The total receivable was $21.8 million and $20.8 million at December 31, 2013 and 2014, respectively, and is included in Other assets. The total payable was $56.8 million and $88.6 million as of December 31, 2013 and 2014, respectively, and is included in Other liabilities. The total amount received for such receivables was $7.1 million, $6.9 million, and $9.7 million for the years ended December 31, 2012, 2013 and 2014, respectively.  The total amount paid for such liabilities was $23.3 million, $5.5 million, and $4.4 million for the years ended December 31, 2012, 2013, 2014, respectively.
	Certain investment partnerships on the Company's balance sheet are co-invested in by a prior owner. The Company has reflected the prior owner's interest in these partnerships as either a liability in Other liabilities or as a Non-controlling interest, depending on the structure of the investment. The total liability was $72.9 million and $71.6 million at December 31, 2013 and 2014, respectively. The total non-controlling interest was $22.2 million and $13.5 million at December 31, 2013 and 2014, respectively.
	In certain cases, Affiliate management owners and Company officers may serve as trustees or directors of certain mutual funds from which the Affiliate earns advisory fee revenue.

Stockholders_Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Stockholders' Equity	Stockholders' Equity
	Preferred Stock
	The Company is authorized to issue up to 5.0 million shares of Preferred Stock. Any such Preferred Stock issued by the Company may rank prior to common stock as to dividend rights, liquidation preference or both, may have full or limited voting rights and may be convertible into shares of common stock.
	Common Stock
	The Company's Board of Directors has authorized the issuance of up to 150.0 million shares of Voting Common Stock and 3.0 million shares of Class B Non-Voting Common Stock. As more fully described in Note 8, the Company is party to a forward equity sale agreement to issue new shares of the Company's common stock.
	The Company's Board of Directors has also authorized share repurchase programs in recent periods. As of December 31, 2014, there were approximately 1.0 million remaining shares available for repurchase under outstanding programs. The timing and amount of issuances and repurchases are determined at the discretion of AMG's management.
	A summary of the Company's recent share repurchase activity is as follows:
	Period	Shares	Average
		Repurchased	Price
	2012	0.6	$	107.44
	2013	0.1	184.89
	2014	1.2	204.72
	The Company has repurchased an additional 0.3 million shares from January 1 through February 18, 2015.
	Financial Instruments
	The Company's junior convertible securities contain an embedded right for holders to receive shares of the Company's common stock under certain conditions. These arrangements, as well as the forward equity sale agreement, meet the definition of equity and are not required to be accounted for separately as derivative instruments.

ShareBased_Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation
	Share-Based Incentive Plans
	The Company has established various plans under which it is authorized to grant restricted stock, restricted stock units, stock options and stock appreciation rights. Compensation payable under these plans is intended to qualify as performance-based compensation within the meaning of Section 162(m) of the Internal Revenue Code. The Company may also grant cash awards that can be notionally invested in one or more specified measurement funds, including the Company’s common stock.
	Share-Based Incentive Compensation
	A summary of recent share-based compensation expense is as follows:
	Period		Share-Based				Tax Benefit
			Compensation
			Expense
	2012		$	25.4			$	9.6
	2013		27.5				10.6
	2014		29.3				11.3
	The cash received for options exercised was $48.2 million and $41.4 million during the years ended December 31, 2013 and 2014, respectively. The actual tax benefit recognized from share-based incentive plans and the excess tax benefit classified as a financing cash flow were $20.7 million and $17.3 million and $60.2 million and $61.5 million during the years ended December 31, 2013 and 2014, respectively.
	There was $79.3 million and $56.8 million of unrecognized compensation expense related to share-based compensation arrangements as of December 31, 2013 and 2014, respectively. The unrecognized compensation expense at December 31, 2014 will be recognized over a weighted average period of approximately three years (assuming no forfeitures).
	Stock Options
	The Company granted stock options with fair values of $2.5 million, $1.0 million, and $0.6 million in 2012, 2013 and 2014, respectively. Stock options generally vest over a period of three to five years and expire seven to ten years after the grant date. All options have been granted with exercise prices equal to the fair market value of the Company's common stock on the date of grant.
	The following table summarizes the transactions of the Company's stock options:
		Stock Options			Weighted				Weighted
					Average				Average
					Exercise				Remaining
					Price				Contractual
									Life (years)
	Unexercised options outstanding—January 1, 2014	3			$	77.71
	Options granted	0			204.99
	Options exercised	(0.7	)		60.47
	Options forfeited	(0.0	)		81.5
	Unexercised options outstanding—December 31, 2014	2.3			83.42				2.7
	Exercisable at December 31, 2014	2.1			80.92				2.6
	For the period January 1 through February 18, 2015, 0.5 million outstanding options have been exercised.
	The fair value of options granted is estimated using the Black-Scholes option pricing model. The weighted average fair value of options granted during the years ended December 31, 2012, 2013 and 2014 was $40.43, $61.82 and $60.20 per option, respectively, based on the weighted-average grant date assumptions stated below.
		For the Years Ended December 31,
		2012			2013			2014
	Dividend yield	0	%		0	%		0	%
	Expected volatility(1)	36.8	%		34.4	%		29.7	%
	Risk-free interest rate(2)	0.7	%		1.5	%		1.8	%
	Expected life of options (in years)(3)	5			5			5
	Forfeiture rate(3)	1.4	%		0	%		0	%
	_______________________________________________________________________________
	-1	Based on historical and implied volatility.
	-2	Based on the U.S. Treasury yield curve in effect at the date of grant.
	-3	Based on the Company's historical data and expected exercise behavior.
	The Company generally uses treasury stock to settle stock option exercises. The total intrinsic value of options exercised during the years ended December 31, 2012, 2013 and 2014 was $86.8 million, $77.4 million and $92.6 million, respectively. As of December 31, 2014, the intrinsic value of exercisable options outstanding was $272.9 million and 3.5 million options are available for future grant under the Company's option plans.
	Restricted Stock
	The Company has granted service-based awards with fair values of $18.1 million, $34.1 million, and $8.0 million in 2012, 2013 and 2014, respectively. These awards were valued based on the closing price of the Company's common stock on the date of grant and contain vesting conditions requiring service over a period of three to eight years. In certain circumstances, the Company may elect to settle the awards in shares of the Company's common stock or cash.
	In 2013, the Company granted awards that vest when a required service period has been completed and if certain market conditions are satisfied. The market conditions require separate 15%, 25% and 35% increases in the price of the Company’s common stock to be achieved during the six-year term of the awards; however, shares underlying the awards are eligible to be delivered, assuming the service conditions have been satisfied, only on the fourth, fifth and sixth anniversaries of the grant date if the 15% market condition has been maintained. The awards were valued using a Monte Carlo simulation with grant date assumptions for expected volatility (31.6%), risk-free interest rate (2.0%) and dividend yield (0.0%). The fair value of the awards was $19.4 million and will be recognized as compensation over a service period of 4 years, which is the longer of the explicit service period or the period the market condition is expected to be met.
	As of December 31, 2014, the Company has 1.7 million shares available for grant under its plans.
	The following table summarizes the transactions of the Company's restricted stock:
		Restricted			Weighted
		Stock			Average
					Grant Date
					Value
	Unvested units—January 1, 2014	0.5			$	176.38
	Units granted	0			190.42
	Units vested	(0.1	)		131.42
	Units forfeited	(0.0	)		173.77
	Unvested units—December 31, 2014	0.4			182.83

Affiliate_Equity

Affiliate Equity	12 Months Ended
	Dec. 31, 2014
Affiliate Equity
Affiliate Equity	Affiliate Equity
	Affiliate equity interests provide holders with a ratable portion of ownership in Affiliates. The Company periodically issues Affiliate equity interests to and repurchases Affiliate equity interests from its Affiliate partners and officers of AMG, with vesting, rights to cash flows and repurchase rights established at the time of grant or sale. Affiliate partners also transfer Affiliate equity interests amongst themselves.
	Sales and repurchases of Affiliate equity generally occur at fair value; however, the Company also grants Affiliate equity to its Affiliate partners, employees and officers as a form of compensation. If the equity is issued for consideration below the fair value of the equity or repurchased for consideration above the fair value of the equity, then such difference is recorded as compensation expense over the requisite service period.
	A summary of Affiliate equity expense is as follows:
	Period		Affiliate Equity Expense			Tax Benefit
	2012		$	60.4		$	17
	2013		72.3			21.7
	2014		84.4			18.2
	Affiliate equity expense attributable to the non-controlling interests was $16.3 million, $15.9 million and $37.0 million in 2012, 2013 and 2014, respectively. As of December 31, 2013 and 2014, the Company had $68.2 million and $71.1 million, respectively, of unrecognized Affiliate equity expense, which will be recognized over a weighted average period of approximately four years (assuming no forfeitures). Of this unrecognized compensation expense, $32.1 million and $41.6 million is attributable to the non-controlling interest, respectively.
	The Company's Affiliate equity arrangements provide the Company a conditional right to call and holders the conditional right to put their retained equity interests at certain intervals. The purchase price of these conditional purchases are generally calculated based upon a multiple of cash flow distributions, which is intended to represent fair value. Holders are also permitted to sell their equity interests to other individuals or entities in certain cases, subject to the Company's approval or other restrictions. The Company, at its option, may pay for Affiliate equity purchases in cash, shares of its common stock or other forms of consideration and can consent to the transfer of these interests to other individuals or entities.
	The current redemption value of these interests has been presented as Redeemable non-controlling interests on the Company's Consolidated Balance Sheets. Changes in the current redemption value are recorded to Additional paid-in capital. The following table presents the changes in Redeemable non-controlling interests during the period:
		2013			2014
	Balance, as of January 1	$	477.5		$	641.9
	Transactions in Redeemable non-controlling interests	(52.5		)	(61.7		)
	Changes in redemption value	216.9			65.3
	Balance, as of December 31	$	641.9		$	645.5
	During the years ended 2012, 2013 and 2014, the Company acquired interests from, and transferred interests to, Affiliate management partners. The following schedule discloses the effect of changes in the Company's ownership interests in its Affiliates on the controlling interest's equity:
		For the Years Ended December 31,
		2012			2013			2014
	Net income (controlling interest)	$	174		$	360.5		$	452.1
	Increase (decrease) in controlling interest paid-in capital from the sale of Affiliate equity	(44.0		)	(74.0		)	(33.3		)
	Change from Net income (controlling interest) and net transfers with non-controlling interests	$	130		$	286.5		$	418.8

Benefit_Plans

Benefit Plans	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Benefit Plans	Benefit Plans
	The Company has three defined contribution plans consisting of a qualified employee profit-sharing plan covering substantially all of its full-time employees and several of its Affiliates, and non-qualified plans for certain senior employees. AMG's other Affiliates generally have separate defined contribution retirement plans. Under each of the qualified plans, AMG and each participating Affiliate, as the case may be, are able to make discretionary contributions for the benefit of qualified plan participants up to Internal Revenue Service limits.
	The Company has established a Deferred Compensation Plan that provides officers and directors of the Company the opportunity to voluntarily defer base salary, bonus payments and director fees, as applicable, on a pre-tax basis, and invest such deferred amounts in one or more specified measurement funds. While the Company has no obligation to do so, the Deferred Compensation Plan also provides the Company the opportunity to make discretionary contributions; in the event any such contributions are made, contributed amounts will be subject to vesting and forfeiture provisions.
	Consolidated expenses related to the Company's benefit plans in 2012, 2013 and 2014 were $12.7 million, $14.1 million and $17.2 million, respectively.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	The consolidated income tax provision includes taxes attributable to the controlling interest and, to a lesser extent, taxes attributable to non-controlling interests as follows:
		For the Years Ended December 31,
		2012				2013				2014
	Controlling Interests:
	Current tax	$	61			$	153.1			$	149.8
	Intangible-related deferred taxes	22.7				38.1				47.8
	Other deferred taxes	(12.1		)		(6.2		)		15.8
	Total controlling interests	71.6				185				213.4
	Non-controlling Interests:
	Current tax	$	11.7			$	13.3			$	15.3
	Deferred taxes	0.5				(4.2		)		(0.8		)
	Total non-controlling interests	12.2				9.1				14.5
	Provision for income taxes	$	83.8			$	194.1			$	227.9
	Income before income taxes (controlling interest)	$	245.6			$	545.5			$	665.5
	Effective tax rate attributable to controlling interests(1)	29.2		%		33.9		%		32.1		%
	_______________________________________________________________________________
	-1	Taxes attributable to the controlling interest divided by Income before income taxes (controlling interest).
	A summary of the consolidated provision for income taxes is as follows:
		For the Years Ended December 31,
		2012				2013				2014
	Current:
	Federal	$	14.5			$	104.1			$	93.8
	State	14.2				21.1				27.1
	Foreign	44				41.2				44.2
	Total current	72.7				166.4				165.1
	Deferred:
	Federal	19.8				38.1				72.5
	State	4.6				8.9				1.1
	Foreign	(13.3		)		(19.3		)		(10.8		)
	Total deferred	11.1				27.7				62.8
	Provision for income taxes	$	83.8			$	194.1			$	227.9
	The components of income before income taxes consisted of the following:
		For the Years Ended December 31,
		2012				2013				2014
	Domestic	$	238.6			$	604			$	784.1
	International	256.6				259.7				229.5
		$	495.2			$	863.7			$	1,013.60
	The Company's effective income tax rate differs from the amount computed by using income before income taxes and applying the U.S. federal income tax rate because of the effect of the following items:
		For the Years Ended December 31,
		2012			2013			2014
	Tax at U.S. federal income tax rate	35	%		35	%		35	%
	State income taxes, net of federal benefit(1)	3.5			2.8			2.1
	Non-deductible expenses	0.7			0.3			0.3
	Valuation allowance(1)	(4.7	)		(0.3	)		—
	Effect of Non-US operations	(3.3	)		(2.5	)		(5.3	)
	Effect of changes in tax law, rates	(2.0	)		(1.4	)		—
	Effect of income from non-controlling interests	(12.2	)		(11.4	)		(9.6	)
		17	%		22.5	%		22.5	%
	_______________________________________________________________________________
	-1	State income taxes includes changes related to state valuation allowances. Valuation allowance includes changes in federal valuation allowances.
	In 2012, the Company reduced its valuation allowance $11.5 million for its foreign loss carryforwards and realized a deferred tax benefit of $7.3 million from the revaluation of its deferred taxes from a reduction in corporate tax rates in the United Kingdom.
	In 2013, the Company realized deferred tax benefits of $5.9 million and $11.2 million on investments in foreign subsidiaries that became permanent in duration and from the revaluation of its deferred taxes from a reduction in corporate tax rates in the United Kingdom, respectively.
	In 2014, the Company reduced its valuation allowance $6.5 million for state net operating loss carryforwards and realized a deferred tax benefit of $24.5 million on investments in foreign subsidiaries that became permanent in duration.
	These changes decreased the controlling interest effective tax rate 6.7%, 2.7% and 4.7% in the years ended December 31, 2012, 2013 and 2014, respectively.
	The Company does not provide for deferred taxes on the excess of the financial reporting basis over the tax basis in its investments in foreign subsidiaries that are permanent in duration. This amount becomes taxable upon a repatriation of assets from a sale or liquidation of the subsidiary. As of December 31, 2014, the amount of such temporary difference was approximately $139.4 million. The deferred taxes not recognized at December 31, 2014 for this temporary difference are approximately $52.4 million.
	The components of deferred tax assets and liabilities are as follows:
		December 31,
		2013				2014
	Deferred Tax Assets
	State net operating loss carryforwards	$	29.3			$	15.2
	Deferred compensation	29.1				26.8
	Tax benefit of uncertain tax positions	16.2				16
	Accrued expenses	17.6				19.6
Foreign loss carryforwards	9.1				11.5
Other	—				1.4
Total deferred tax assets	101.3				90.5
Valuation allowance	(36.6		)		(18.4		)
Deferred tax assets, net of valuation allowance	$	64.7			$	72.1
Deferred Tax Liabilities
Intangible asset amortization	$	(241.3	)		$	(270.9	)
Convertible securities interest	(144.7		)		(92.5		)
Non-deductible intangible amortization	(101.5		)		(126.4		)
Deferred revenue	(32.1		)		(74.0		)
Other	(2.0		)		—
Total deferred tax liabilities	(521.6		)		(563.8		)
Net deferred tax liability	$	(456.9	)		$	(491.7	)
Deferred tax liabilities are primarily the result of tax deductions for the Company's intangible assets and convertible securities. The Company amortizes most of its intangible assets for tax purposes only, reducing its tax basis below its carrying value for financial statement purposes and generating deferred taxes each reporting period. The Company's convertible securities also generate deferred taxes because the Company's tax deductions are higher than the interest expense recorded for financial statement purposes.
At December 31, 2014, the Company has state net operating loss carryforwards which expire over an 18-year period and foreign loss carryforwards which expire over a 20-year period. The Company has established a valuation allowance to offset these carryforwards because of the uncertainty of realizing their full value.
In 2012 and 2013, the Company reduced its valuation allowance $11.5 million and $1.6 million, respectively from improved projections of taxable income.
In 2014, the Company reduced its valuation allowance $15.7 million on state net operating loss carryforwards from improved projections of taxable income, and increased state uncertain tax positions by $9.2 million for a net $6.5 million state tax benefit.
The Company had uncertain tax positions of $22.6 million, $20.4 million and $28.8 million as of December 31, 2012, 2013 and 2014, respectively. These amounts included $2.2 million, $1.7 million and $1.6 million of interest and related charges, respectively. At December 31, 2012, 2013 and 2014, these liabilities also included $19.4 million, $17.2 million and $26.4 million, respectively, for tax positions that, if recognized, would affect the Company's effective tax rate.
A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:
	For the Years Ended December 31,
	2012				2013				2014
Balance, as of January 1	$	21.2			$	22.6			$	20.4
Additions based on current year tax positions	5.9				4.1				2.6
Additions based on prior years' tax positions	5.2				—				10.8
Reductions for prior years' tax provisions	(5.4		)		(0.1		)		—
Reductions related to lapses of statutes of limitations	(4.6		)		(5.4		)		(4.1		)
Additions (reductions) related to foreign exchange rates	0.3				(0.8		)		(0.9		)
Balance, as of December 31	$	22.6			$	20.4			$	28.8
During 2012, uncertain tax positions decreased $5.4 million as a result of the recognition of tax benefits from the transfer of interests in an Affiliate. This decrease was offset by a $5.2 million provision for added foreign exposure for prior years. During 2014, uncertain tax positions increased $10.8 million, principally for the uncertain nature of several of its state net operating loss carryforwards. The Company does not anticipate that this liability will change significantly over the next twelve months.
The Company periodically has tax examinations in the U.S. and foreign jurisdictions. Examination outcomes, and any related settlements, are subject to significant uncertainty. The completion of examinations may result in the payment of additional taxes and/or the recognition of tax benefits. The Company is generally no longer subject to income tax examinations by any tax authorities for years before 2009.

Earnings_Per_Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share
	The following is a reconciliation of the numerator and denominator used in the calculation of basic and diluted earnings per share available to common stockholders.
		For the Years Ended December 31,
		2012			2013			2014
	Numerator
	Net income (controlling interest)	$	174		$	360.5		$	452.1
	Interest expense on convertible securities, net of taxes	—			10.5			15.2
	Net income (controlling interest), as adjusted	$	174		$	371		$	467.3
	Denominator
	Average shares outstanding (basic)	51.7			53.1			55
	Effect of dilutive instruments:
	Stock options and other awards	1.3			1.3			1.2
	Forward sale	—			0.3			0
	Junior convertible securities	—			2			2.2
	Average shares outstanding (diluted)	53			56.7			58.4
	During the years ended December 31, 2012, 2013 and 2014, the Company repurchased approximately 0.6 million, 0.1 million and 1.2 million shares of common stock, respectively, at an average share price of $107.44, $184.89 and $204.72, respectively, under the share repurchase programs approved by the Company's Board of Directors.
	The diluted earnings per share calculations in the table above exclude the anti-dilutive effect of the following shares:
		For the Years Ended December 31,
		2012		2013			2014
	Stock options and other awards	0.7		0.1			0
	Senior convertible securities	3.6		2.1			—
	Junior convertible securities	4.2		2.2			0.4
	Forward equity sales	0.2		—			—
	As discussed further in Note 20, the Company may settle portions of its Affiliate equity purchases in shares of its common stock. Because it is the Company's intent to settle these potential repurchases in cash, the calculation of diluted earnings per share excludes any potential dilutive effect from possible share settlements.

Comprehensive_Income

Comprehensive Income	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Comprehensive Income	Comprehensive Income
	The following table shows the tax effects allocated to each component of Other comprehensive income:
		For the Year Ended December 31, 2012
		Pre-Tax			Tax Benefit (Expense)			Net of Tax
	Foreign currency translation adjustment	$	18.5		$	—		$	18.5
	Change in net realized and unrealized gain (loss) on derivative securities	(1.1		)	0.4			(0.7		)
	Change in net unrealized gain (loss) on investment securities	21.6			(8.1		)	13.5
	Other comprehensive income (loss)	$	39		$	(7.7	)	$	31.3
		For the Year Ended December 31, 2013
		Pre-Tax			Tax Benefit (Expense)			Net of Tax
	Foreign currency translation adjustment	$	(19.6	)	$	—		$	(19.6	)
	Change in net realized and unrealized gain (loss) on derivative securities	1.5			(0.5		)	1
	Change in net unrealized gain (loss) on investment securities	19.5			(8.0		)	11.5
	Other comprehensive income (loss)	$	1.4		$	(8.5	)	$	(7.1	)
		For the Year Ended December 31, 2014
		Pre-Tax			Tax Benefit (Expense)			Net of Tax
	Foreign currency translation adjustment	$	(62.0	)	$	—		$	(62.0	)
	Change in net realized and unrealized gain (loss) on derivative securities	0.6			(0.3		)	0.3
	Change in net unrealized gain (loss) on investment securities	5.6			(2.2		)	3.4
	Other comprehensive income (loss)	$	(55.8	)	$	(2.5	)	$	(58.3	)
	The components of Accumulated other comprehensive income, net of taxes, are as follows:
		Foreign Currency Translation Adjustment			Realized and Unrealized Losses on Derivative Securities			Unrealized Gain (Loss) on Investment Securities			Total
	Balance, as of December 31, 2013	$	56.6		$	(1.9	)	$	19.5		$	74.2
	Other comprehensive income (loss) before reclassifications	(62.0		)	0.3			1.3			(60.4		)
	Amounts reclassified from other comprehensive income	—			0			2.1			2.1
	Net other comprehensive income (loss)	(62.0		)	0.3			3.4			(58.3		)
	Balance, as of December 31, 2014	$	(5.4	)	$	(1.6	)	$	22.9		$	15.9

Selected_Quarterly_Financial_D

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	Selected Quarterly Financial Data (Unaudited)
	The following is a summary of the quarterly results of operations of the Company for the years ended December 31, 2013 and 2014.
		2013
		First		Second		Third		Fourth
		Quarter		Quarter		Quarter		Quarter
	Revenue	$	502.2	$	541	$	551.6	$	594
	Operating income	150.5		161.4		169.1		153.1
	Income before income taxes	167.3		172.4		187.4		336.6
	Net income (controlling interest)	62.4		64.7		75.2		158.2
	Earnings per share (diluted)	$	1.15	$	1.18	$	1.37	$	2.79
		2014
		First		Second		Third		Fourth
		Quarter		Quarter (1)		Quarter (1)		Quarter
	Revenue	$	593.1	$	636.3	$	640.3	$	641.2
	Operating income	193.9		198.4		224.2		198.7
	Income before income taxes	208.2		239.1		253.4		312.8
	Net income (controlling interest)	77.2		99.1		103.2		172.6
	Earnings per share (diluted)	$	1.4	$	1.75	$	1.82	$	3.02
	_______________________________________________________________________________
	-1	In the fourth quarter of 2014, the Company finalized its purchase price allocation for its investment in EIG.  The final purchase price allocation resulted in a change in Income from equity method investments and Intangible-related deferred taxes in the second and third quarters of 2014.  Income before taxes decreased $1.6 million and $1.7 million in the second and third quarters, respectively, Net income (controlling interest) decreased $1.0 million and $1.1 million in the second and third quarters, respectively, and Earnings per share (diluted) decreased $0.02 in both periods.

Segment_Information

Segment Information	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Segment Information	Segment Information
	Management has assessed and determined that the Company operates in three business segments representing the Company's three principal distribution channels: Institutional, Mutual Fund and High Net Worth, each of which has different client relationships. The following table summarizes the Company's financial results for each of the distribution channels:
	Statements of Income
		As of and for the Year Ended December 31, 2012
		Institutional		Mutual Fund		High Net Worth		Total
	Revenue	$	861.3	$	774.4	$	169.8	$	1,805.50
	Net income (controlling interest)	126		27.3		20.7		174
	Total assets	3,176.50		2,354.80		655.8		6,187.10
	Goodwill	1,078.50		939.5		337.2		2,355.20
	Equity method investments in Affiliates	879.6		81.2		70.5		1,031.30
		As of and for the Year Ended December 31, 2013
		Institutional		Mutual Fund		High Net Worth		Total
	Revenue	$	948.7	$	1,023.00	$	217.1	$	2,188.80
	Net income (controlling interest)	219.9		103.4		37.2		360.5
	Total assets	3,196.50		2,448.40		673.9		6,318.80
	Goodwill	1,076.30		928.1		337.3		2,341.70
	Equity method investments in Affiliates	942.6		77.7		103		1,123.30
		As of and for the Year Ended December 31, 2014
		Institutional		Mutual Fund		High Net Worth		Total
	Revenue	$	1,022.80	$	1,242.60	$	245.5	2,510.90
	Net income (controlling interest)	227		180.1		45		452.1
	Total assets	3,739.80		3,082.00		876.3		7,698.10
	Goodwill	1,159.10		1,125.30		368.4		2,652.80
	Equity method investments in Affiliates	1,533.80		150.3		99.4		1,783.50
	In 2012, 2013 and 2014, revenue attributable to clients domiciled outside the U.S. was approximately 41%, 38% and 37% of total revenue, respectively.

Schedule_II_Valuation_and_Qual

Schedule II Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2014
Valuation and Qualifying Accounts [Abstract]
Schedule II Valuation and Qualifying Accounts	Schedule II
	Valuation and Qualifying Accounts
	(in millions)	Balance		Additions		Additions		Deductions		Balance
		Beginning of		Charged to Costs		Charged to				End of Period
		Period		and Expenses		Other Accounts
	Income Tax Valuation Allowance
	Year Ending December 31,
	2014	$	36.6	$	—	$	—	$	18.2	$	18.4
	2013	21.3		16.9		—		1.6		36.6
	2012	35.6		—		—		14.3		21.3
	Other Allowances(1)
	Year Ending December 31,
	2014	$	8.8	$	4.7	$	—	$	1.4	$	12.1
	2013	8.4		2.8		—		2.4		8.8
	2012	9.6		0.1		—		1.3		8.4
	_______________________________________________________________________________
	-1	Other Allowances represents reserves on notes received in connection with transfers of our interests in certain Affiliates as well as other receivable amounts, which we consider uncollectible.

Business_and_Summary_of_Signif1

Business and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation
	The financial statements are prepared in accordance with accounting principles generally accepted in the U.S. ("GAAP"). All dollar amounts, except per share data in the text and tables herein, are stated in millions unless otherwise indicated. Certain reclassifications have been made to prior years' financial statements to conform to the current year's presentation.
Use of Estimates	Use of Estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts and disclosures in the financial statements. Actual results could differ from those estimates.
Principles of Consolidation	Principles of Consolidation
	Investments in Affiliates
	The Company evaluates the risk, rewards, and significant terms of each of its Affiliate and other investments to determine the appropriate method of accounting. Majority-owned or otherwise controlled investments are consolidated. AMG typically has control over its Affiliate investments, directly or indirectly, as the sole general partner (in the case of Affiliates which are limited partnerships), the managing partner (in the case of Affiliates which are limited liability partnerships), the sole manager member (in the case of Affiliates which are limited liability companies) or principal shareholder (in the case of Affiliates which are corporations). As a result, the Company generally consolidates its Affiliate investments and all material intercompany balances and transactions are eliminated.
	For its consolidated Affiliates, the portion of the Owners’ Allocation allocated to Affiliate partners other than AMG is included in Net income (non-controlling interests) in the Consolidated Statements of Income. Non-controlling interests on the Consolidated Balance Sheets includes capital and undistributed Operating and Owners’ Allocation owned by Affiliate partners of the consolidated Affiliates. The effect of any changes in the Company's equity interests in its consolidated Affiliates resulting from the issuance or repurchase of an Affiliate's equity by the Company or one of its Affiliates is included as a component of stockholders' equity, net of the related income tax effect in the period of the change. The current redemption value of non-controlling interests has been presented as Redeemable non-controlling interests on the Consolidated Balance Sheets.
	AMG applies the equity method of accounting to investments where AMG does not hold a controlling equity interest but has the ability to exercise significant influence over operating and financial matters. In cases where AMG applies the equity method of accounting, it typically does not have an obligation to repurchase equity interests. Other investments in which AMG owns less than a 20% interest and does not exercise significant influence are accounted for under the cost method. Under the cost method, income is recognized as dividends when, and if, declared.
	Affiliate Sponsored Investment Vehicles
	The Company's Affiliates sponsor various investment vehicles where they also act as the investment advisor. Certain of these investment vehicles are variable interest entities (“VIEs”) while others are voting rights entities ("VREs").
	VIEs are consolidated if the Affiliate is determined to be the primary beneficiary (i.e. if it absorbs a majority of the expected losses, or receives a majority of the expected residual returns). In determining whether the Affiliate is the primary beneficiary, both qualitative and quantitative factors (e.g. the voting rights of the equity holders, economic participation of all parties, including how fees are earned and paid, related party ownership, guarantees and implied relationships) are considered.
	VREs are consolidated if the Affiliate is the managing member or general partner of the investment vehicle unless unaffiliated investors have certain rights to remove the Affiliate from such role or have substantive participating rights(e.g. approval of annual operating budgets, major financings, selection of senior management).
Cash and Cash Equivalents	Cash and Cash Equivalents
	The Company considers all highly liquid investments, including money market mutual funds, with original maturities of three months or less to be cash equivalents. Cash equivalents are stated at cost, which approximates market value due to the short-term maturity of these investments.
Receivables	Receivables
	The Company’s Affiliates earn advisory and performance fees which are billed monthly, quarterly or annually based on the terms of the related contracts. Billed but uncollected advisory and performance fees are presented within Receivables on the Consolidated Balance Sheets and are generally short-term in nature.
	Certain of the Company’s Affiliates in the United Kingdom act as an intermediary between clients and their sponsored funds. Normal settlement periods on transactions initiated by these clients result in unsettled fund share receivables and payables that are presented on a gross basis within Receivables and Payables and accrued liabilities on the Consolidated Balance Sheets. The gross presentation of these receivables and offsetting payables reflect the legal relationship between the underlying investor and the Company’s Affiliates.
Investments in Marketable Securities	Investments in Marketable Securities
	Investments in marketable securities are classified as either trading or available-for-sale and carried at fair value. Unrealized gains or losses on investments classified as available-for-sale are reported, net of tax, as a separate component of Accumulated other comprehensive income in Equity until realized. Unrealized gains or losses related to trading securities are reported within Investment and other income in the period they occur. If a decline in the fair value of an available-for-sale investment is determined to be other than temporary, the carrying amount of the asset is reduced to its fair value, and the difference is charged to income in the period incurred.
Fair Value Measurements	Fair Value Measurements
	The Company determines the fair value of certain investment securities and other financial and non-financial assets and liabilities. Fair value is determined based on the price that would be received for an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date, utilizing a hierarchy of three different valuation techniques:
	Level 1 - Unadjusted quoted market prices for identical instruments in active markets;
	Level 2 - Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations whose inputs, or significant value drivers, are observable; and
	Level 3 - Prices reflect the Company's own assumptions concerning unobservable inputs to the valuation model. These inputs require significant management judgment and reflect the Company's assumptions that market participants would use in pricing the asset or liability.
Fixed Assets	Fixed Assets
	Fixed assets are recorded at cost and depreciated using the straight-line method over their estimated useful lives. The estimated useful lives of office equipment and furniture and fixtures range from three to ten years. Computer software developed or obtained for internal use is amortized using the straight-line method over the estimated useful life of the software, generally three to seven years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the term of the lease, and the building is amortized over 39 years. The costs of improvements that extend the life of a fixed asset are capitalized, while the cost of repairs and maintenance are expensed as incurred. Land is not depreciated.
Leases	Leases
	The Company and its Affiliates currently lease office space and equipment under various leasing arrangements. As these leases expire, it can be expected that in the normal course of business they will be renewed or replaced. Leases are classified as either capital leases or operating leases, as appropriate. Most lease agreements classified as operating leases contain renewal options, rent escalation clauses or other inducements provided by the landlord. Rent expense is accrued to recognize lease escalation provisions and inducements provided by the landlord, if any, on a straight-line basis over the lease term.
Equity Investments in Affiliates	Equity Investments in Affiliates
	For equity method investments, the Company's portion of income or loss before taxes is included in Income from equity method investments, net of any amortization of intangible assets related to the Company's investment. The Company's share of income taxes incurred directly by Affiliates accounted for under the equity method are recorded within income taxes because these taxes generally represent the Company's share of the taxes incurred by the Affiliate.
	The Company periodically evaluates its equity method investments for impairment. In such impairment evaluations, the Company assesses if the fair value of the investment has declined below its carrying value for a period considered to be other than temporary. If the Company determines that a decline in fair value below the carrying value of the investment is other than temporary, then the reduction in carrying value would be recognized in Income from equity method investments in the Consolidated Statements of Income.
Acquired Client Relationships and Goodwill	Acquired Client Relationships and Goodwill
	Each acquired Affiliate has identifiable assets arising from contractual or other legal rights with their clients ("acquired client relationships"). In determining the value of acquired client relationships, the Company analyzes the net present value of each acquired Affiliate's existing client relationships based on a number of factors including: the Affiliate's historical and potential future operating performance; the Affiliate's historical and potential future rates of attrition among existing clients; the stability and longevity of existing client relationships; the Affiliate's recent, as well as long-term, investment performance; the characteristics of the firm's products and investment styles; the stability and depth of the Affiliate's management team and the Affiliate's history and perceived franchise or brand value.
	The Company has determined that certain of its mutual fund acquired client relationships meet the criteria to be considered indefinite-lived assets because the Company expects both the renewal of these contracts and the cash flows generated by these assets to continue indefinitely. Accordingly, the Company does not amortize these intangible assets, but instead reviews these assets annually or more frequently whenever events or circumstances occur indicating that the recorded indefinite-lived assets may be impaired. Each reporting period, the Company assesses whether events or circumstances have occurred which indicate that the indefinite life criteria are no longer met. If the indefinite life criteria are no longer met, the Company would assess whether the carrying value of the assets exceeds its fair value, an impairment loss would be recorded in an amount equal to any such excess and these assets would be reclassified to definite-lived.
	The expected useful lives of definite-lived acquired client relationships are analyzed each period and determined based on an analysis of the historical and projected attrition rates of each Affiliate's existing clients, and other factors that may influence the expected future economic benefit the Company will derive from the relationships.
	The Company tests for the possible impairment of indefinite and definite-lived intangible assets annually or more frequently whenever events or changes in circumstances indicate that the carrying amount of the asset is not recoverable. If such indicators exist, the Company compares the fair value of the asset to the carrying value of the asset. If the carrying value is greater than the fair value, an impairment loss may be recorded.
	Goodwill represents the future economic benefits arising from assets acquired in a business combination that are not individually identified and separately recognized, and is reported within the segments in which the Affiliate operates. Goodwill is not amortized, but is instead reviewed for impairment. The Company assesses goodwill for impairment at least annually, or more frequently whenever events or circumstances occur indicating that the recorded goodwill may be impaired. If the carrying amount of goodwill exceeds the fair value, an impairment loss may be recorded.
Issuance Costs	Issuance Costs
	Issuance costs related to our senior bank debt are amortized over the remaining term of the credit facility and term loan. Costs incurred to issue debt are amortized over the shorter of the period to the first investor put or the Company's estimate of the expected term of the security. Costs associated with financial instruments that are not required to be accounted for separately as derivative instruments are charged directly to stockholders' equity.
Derivative Financial Instruments	Derivative Financial Instruments
	The Company utilizes financial instruments, specifically interest rate derivative contracts to hedge certain interest rate exposures. In entering into these contracts, the Company intends to offset cash flow gains and losses that occur on its existing debt obligations with cash flow gains and losses on the contracts hedging these obligations.
	From time to time, the Company's Affiliates use foreign currency forward contracts to hedge the risk of currency exchange rate movements. In entering into these contracts, the Affiliates intend to offset cash flow gains and losses on projected foreign currency-denominated revenues and expenses as a result of variability in foreign currency exchange rates.
	The Company records all derivatives on the balance sheet at fair value. If the Company's derivatives qualify as cash flow hedges, the effective portion of the unrealized gain or loss is recorded in Accumulated other comprehensive income as a separate component of stockholders' equity and reclassified into earnings when the hedged cash flows are recorded in earnings. Hedge effectiveness is generally measured by comparing the present value of the cumulative change in the expected future variable cash flows of the hedged contract with the present value of the cumulative change in the expected future variable cash flows of the hedged item. To the extent that the critical terms of the hedged item and the derivative are not identical, hedge ineffectiveness would be reported in Investment and other income. If the Company's derivatives do not qualify as cash flow or fair value hedges, changes in the fair value of the derivatives are recognized as a gain or loss in Investment and other income.
Contingent Payment Arrangements	Contingent Payment Arrangements
	The Company periodically enters into contingent payment arrangements in connection with its business combinations. In these arrangements, the Company agrees to pay additional consideration to the sellers to the extent that certain levels of revenue growth are achieved. For consolidated Affiliates, the Company estimates the fair value of these potential future obligations at the time a business combination is consummated and records a liability on its Consolidated Balance Sheet. The Company then accretes the obligation to its expected payment amount over the period until the arrangement is measured. If the Company's expected payment amount subsequently changes, the obligation is reduced or increased in the current period resulting in a gain or loss, respectively. Both gains and losses resulting from changes to expected payments and the accretion of these obligations to their expected payment amounts are reflected within Imputed interest expense and contingent payment arrangements in the Company's Consolidated Statements of Income. For Affiliates accounted for under the equity method, the Company records a liability when a payment becomes probable with a corresponding increase to the carrying value of the Affiliate.
Income Taxes	Income Taxes
	The Company accounts for income taxes using the liability method. Under this method, deferred taxes are recognized for the expected future tax consequences of temporary differences between the book carrying amounts and tax bases of the Company's assets and liabilities. Deferred tax liabilities are generally attributable to intangible assets, convertible securities and deferred revenue. Deferred tax assets are generally attributable to state and foreign loss carryforwards, deferred compensation and the benefit of uncertain tax positions. Intangible-related deferred tax liabilities are not recorded on U.S. basis differences related to the Company's foreign investments in corporate stock because of the permanent nature of the Company's investments.
	In measuring the amount of deferred taxes each period, the Company must project the impact on its future tax payments of any reversal of deferred tax liabilities (which would increase the Company's tax payments), and any use of its state and foreign loss carryforwards (which would decrease its tax payments). In forming these estimates, the Company uses enacted federal, state and foreign income tax rates and makes assumptions about the apportionment of future taxable income to jurisdictions in which the Company has operations. An increase or decrease in foreign, federal or state income tax rates could have a material impact on the Company's deferred income tax liabilities and assets and would result in a current income tax charge or benefit.
	The Company recognizes the financial statement benefit of an uncertain tax position only after considering the probability that a tax authority would sustain the position in an examination. For tax positions meeting a "more-likely-than-not" threshold, the amount recognized in the financial statements is the benefit expected to be realized upon settlement with the tax authority. For tax positions not meeting the threshold, no financial statement benefit is recognized. The Company recognizes interest and other charges relating to unrecognized tax benefits as additional tax expense.
	In the case of the Company's deferred tax assets, the Company regularly assesses the need for valuation allowances, which would reduce these assets to their recoverable amounts. In forming these estimates, the Company makes assumptions of future taxable income that may be generated to utilize these assets, which have limited lives. If the Company determines that these assets will be realized, the Company records an adjustment to the valuation allowance, which would decrease tax expense in the period such determination was made. Likewise, should the Company determine that it would be unable to realize additional amounts of deferred tax assets, an adjustment to the valuation allowance would be charged to tax expense in the period such determination was made.
Foreign Currency Translation	Foreign Currency Translation
	The assets and liabilities of Affiliates whose functional currency is not the U.S. dollar are translated into U.S. dollars using exchange rates in effect as of the balance sheet date. The revenue and expenses of these Affiliates are translated into U.S. dollars using average exchange rates for the relevant period. Because of the permanent nature of the Company's investments, net translation exchange gains and losses are excluded from Net income but are recorded in Other comprehensive income. Foreign currency transaction gains and losses are reflected in Investment and other income.
Concentration of Credit Risk	Concentration of Credit Risk
	Financial instruments that potentially subject the Company to significant concentrations of credit risk consist principally of cash investments. The Company maintains cash and cash equivalents, investments and, at times, certain financial instruments with various high credit-quality financial institutions. These financial institutions are typically located in countries in which AMG and its Affiliates operate. For AMG and certain Affiliates, cash deposits at a financial institution may exceed Federal Deposit Insurance Corporation insurance limits.
Revenue Recognition	Revenue Recognition
	The Company's consolidated revenue primarily represents advisory fees billed by Affiliates for managing the assets of clients. Asset-based advisory fees are recognized as services are rendered and are based upon a percentage of the value of client assets managed. Any fees collected in advance are deferred and recognized as income over the period earned. Performance-based advisory fees are generally assessed as a percentage of the investment performance realized on a client's account, generally over an annual period. Performance-based advisory fees are recognized when they are earned (i.e. when they become billable to customers and are not subject to contingent repayment) based on the contractual terms of agreements and when collection is reasonably assured. Carried interest is recognized upon the earlier of the termination of the investment product or when the likelihood of claw-back is improbable. Also included in revenue are commissions earned by broker dealers, recorded on a trade date basis, and other service fees recorded as earned.
	The Company's Affiliates have contractual arrangements with third parties to provide certain distribution-related services. These third parties are primarily compensated based on the value of client assets over time. Distribution related revenues are presented gross of any related expenses when the Affiliate is the principal in its role as primary obligor under its sales and distribution arrangements. Distribution-related expenses are presented within Selling, general and administrative expenses.
Earnings Per Share	Earnings Per Share
	The calculation of basic earnings per share is based on the weighted average number of shares of the Company's common stock outstanding during the period. Diluted earnings per share is similar to basic earnings per share, but adjusts for the dilutive effect of the potential issuance of incremental shares of the Company's common stock.
	The Company had convertible securities outstanding during the periods presented and is required to apply the if-converted method to these securities in its calculation of diluted earnings per share. Under the if-converted method, shares that are issuable upon conversion are deemed outstanding, regardless of whether the securities are contractually convertible into the Company's common stock at that time. For this calculation, the interest expense (net of tax) attributable to these dilutive securities is added back to Net income (controlling interest), reflecting the assumption that the securities have been converted. Issuable shares for these securities and related interest expense are excluded from the calculation if an assumed conversion would be anti-dilutive to diluted earnings per share.
Share-Based Compensation Plans	Share-Based Compensation Plans
	The Company recognizes expenses for all share-based payments based on their grant date fair values over the requisite service period.
	The Company reports any tax benefits realized upon the exercise of stock options that are in excess of the expense recognized for reporting purposes as a financing activity in the Company's Consolidated Statements of Cash Flows. If the tax benefit realized is less than the expense, the tax shortfall is recognized in stockholders' equity. To the extent the expense exceeds available windfall tax benefits, it is recognized in the Consolidated Statements of Income. The Company was permitted to calculate its cumulative windfall tax benefits for the purposes of accounting for future tax shortfalls. The Company elected to apply the long-form method for determining the pool of windfall tax benefits.
Segment Information	Segment Information
	Management has assessed and determined that the Company operates in three business segments representing the Company's three principal distribution channels: Institutional, Mutual Fund and High Net Worth, each of which has different client relationships.
	Revenue in the Institutional distribution channel is earned from relationships with public and private client entities, including pension plans, foundations, endowments and sovereign wealth funds. Revenue in the Mutual Fund distribution channel is earned from advisory and sub-advisory relationships with all domestically-registered investment products as well as investment products that are registered abroad. Revenue in the High Net Worth distribution channel is earned from relationships with ultra-high and high net worth individuals, family trusts and managed account programs.
	Revenue earned from client relationships managed by Affiliates accounted for under the equity method is not consolidated with the Company's reported Revenue but instead is included (net of operating expenses, including amortization) in Income from equity method investments, and reported in the distribution channel in which the Affiliate operates.
	In reporting segment operating expenses, consolidated Affiliate expenses are allocated to a particular segment on a pro rata basis with respect to the revenue generated by that Affiliate in such segment. Generally, as revenue increases, additional compensation is typically paid to Affiliate management partners from the Operating Allocation. As a result, the contractual expense allocation pursuant to a revenue sharing arrangement may result in the characterization of any growth in margin beyond the Company's Owners' Allocation as an operating expense. Unless specific to a distribution channel, all other operating and non-operating expenses have been allocated to segments based on the proportion of cash flow distributions reported by Affiliates in each segment.
Recent Accounting Developments	Recent Accounting Developments
	In April 2014, the Financial Accounting Standards Board (the "FASB") issued an update to the guidance for discontinued operations accounting and reporting. The new guidance amends the definition of discontinued operations and requires entities to provide additional disclosures regarding disposal transactions that do not meet the discontinued operations criteria. The new guidance is effective for interim and fiscal periods beginning after December 15, 2014. The Company does not expect this guidance to have a significant impact on the Consolidated Financial Statements.
	In May 2014, the FASB issued a final standard on revenue from contracts with customers. The new standard provides a comprehensive model for revenue recognition. The new standard is effective for interim and fiscal periods beginning after December 15, 2016. The Company is evaluating the impact of this standard on its Consolidated Financial Statements.
	In November 2014, the FASB issued an update to the guidance for hybrid financial instruments. The new guidance requires entities to determine the nature of the host contract by considering all stated and implied substantive terms and features of a hybrid financial instrument, weighing each term and feature on the basis of relevant facts and circumstances. The new guidance is effective for interim and fiscal periods beginning after December 15, 2015. The Company does do not expect this guidance to have a significant impact on its Consolidated Financial Statements.
	In February 2015, the FASB issued a new standard that amended the current consolidation guidance. The new standard changes the analysis required to determine whether an entity is a variable interest entity and should be consolidated. The new standard is effective for interim and fiscal periods beginning after December 15, 2015, and early adoption is permitted. The Company is evaluating the impact of this new standard on its Consolidated Financial Statements.

Investments_in_Marketable_Secu1

Investments in Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2014
Marketable Securities [Abstract]
Summary of the Cost, Gross Unrealized Gains and Losses, and Fair Value of Investments	The following is a summary of the cost, gross unrealized gains and losses and fair value of investments classified as available-for-sale and trading at December 31, 2013 and 2014:
		Available-for-Sale						Trading
		31-Dec-13			31-Dec-14			31-Dec-13			December 31, 2014
	Cost	$	103.2		$	125.6		$	17.9		$	19.5
	Unrealized Gains	33.3			42.8			4.6			2.9
	Unrealized Losses	(1.1		)	(18.1		)	(0.0		)	(0.1		)
	Fair Value	$	135.4		$	150.3		$	22.5		$	22.3

Variable_Interest_Entities_Tab

Variable Interest Entities (Tables)	12 Months Ended
	Dec. 31, 2014
Variable Interest Entities
Schedule of Net Assets and Liabilities and Maximum Risk of Losses Related to Unconsolidated VIEs	The net assets and liabilities of these unconsolidated VIEs and the Company's maximum risk of loss are as follows:
		31-Dec-13				31-Dec-14
	Category of Investment	Unconsolidated		Carrying Value and		Unconsolidated		Carrying Value and
		VIE Net Assets		Maximum Exposure		VIE Net Assets		Maximum Exposure
				to Loss				to Loss
	Sponsored investment funds	$	8,112.70	$	1.7	$	8,550.40	$	1.2

Senior_Notes_Tables

Senior Notes (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Summary of Principal Terms of Senior Notes	At December 31, 2014, the Company had three senior notes outstanding. The principal terms of these notes are summarized below.
		2022			2024			2042
		Senior			Senior			Senior
		Notes			Notes			Notes
	Issue date	Oct-12			Feb-14			Aug-12
	Maturity date	Oct-22			Feb-24			Aug-42
	Potential Call Date	Oct-15			Any Time (1)			Aug-17
	Par value (in millions)	$	140		$	400		$	200
	Call Price	At Par			As Defined (1)			At Par
	Stated coupon	5.25		%	4.25		%	6.375		%
	Coupon frequency	Quarterly			Semi-annually			Quarterly
	_______________________________________________________________________________
	-1	The 2024 senior notes may be redeemed at any time, in whole or in part, at a make-whole redemption price plus accrued and unpaid interest.

Convertible_Securities_Tables

Convertible Securities (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Schedule of Carrying Value of Convertible Securities	The carrying values of the Company's convertible securities are as follows:
		31-Dec-13				31-Dec-14
		Carrying		Principal amount		Carrying		Principal amount
		Value		at maturity		Value		at maturity
	Junior convertible securities:
	2007 junior convertible securities(1)	$	301.2	$	430.8	$	303.1	$	430.8
	2006 junior convertible securities	217.5		300		—		—
	Total junior convertible securities	$	518.7	$	730.8	$	303.1	$	430.8
	_______________________________________________________________________________
	-1	The carrying value is accreted to the principal amount at maturity over a remaining life of 23 years.

Derivative_Financial_Instrumen1

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Interest Rate Swap Agreements	In 2010, the Company entered into interest rate swap agreements as summarized in the table below:
		Notional			Paying			Receiving		Start Date		Expiration Date
		Amount
	Counterparty A	$	25		1.67	%		3-Month LIBOR		Oct-10		Oct-15
	Counterparty A	$	25		1.65	%		3-Month LIBOR		Oct-10		Oct-15
	Counterparty B	$	25		1.59	%		3-Month LIBOR		Oct-10		Oct-15
	Counterparty B	$	25		2.14	%		3-Month LIBOR		Oct-10		Oct-17
Schedule of Foreign Currency Forward Contracts	In 2014, one of the Company's Affiliates entered into foreign currency forward contracts relating to projected expenses denominated in pound sterling (GBP) and projected revenues denominated in euro (EUR):
			Paying			Receiving				Start Date		Expected Settlement
	Counterparty C		$	21.9		£	13.5			Sep-14		Monthly 2015
	Counterparty C		€	28		$	34.3			Dec-14		Quarterly 2015
Schedule of Derivative Instrument Gains and (Losses) before Taxes	The following summarizes the amount of derivative instrument gains and losses reported in the Consolidated Statements of Comprehensive Income:
		For the Years Ended December 31,
	Cash Flow Hedges	2012			2013				2014
	Interest rate swaps	$	(1.1	)	$	1.5			$	1.1
	Treasury rate locks	0			0				0
	Foreign currency forward contracts	—			—				(0.5		)
	Total	$	(1.1	)	$	1.5			$	0.6

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Schedule of Fair Value of Financial Assets and Liabilities Measured on a Recurring Basis	The following table summarizes the Company's financial assets and liabilities that are measured at fair value on a recurring basis:
						Fair Value Measurements
		31-Dec-13
						Level 1				Level 2				Level 3
	Financial Assets
	Cash equivalents	$	39			$	39			$	—			$	—
	Investments in marketable securities(1)
	Trading securities	22.5				22.5				—				—
	Available-for-sale securities	135.4				135.4				—				—
	Other investments	164.3				14.1				18.4				131.8
	Financial Liabilities(2)
	Contingent payment arrangements	$	50.2			$	—			$	—			$	50.2
	Obligations to related parties	76.9				—				—				76.9
	Interest rate swaps	2.5				—				2.5				—
						Fair Value Measurements
		31-Dec-14
						Level 1				Level 2				Level 3
	Financial Assets
	Cash equivalents	$	59.1			$	59.1			$	—			$	—
	Investments in marketable securities(1)
	Trading securities	22.3				22.3				—				—
	Available for sale securities	150.3				150.3				—				—
	Other investments	167.2				13.6				19.4				134.2
	Financial Liabilities(2)
	Contingent payment arrangements	$	59.3			$	—			$	—			$	59.3
	Obligations to related parties	93.1				—				—				93.1
	Interest rate swaps	1.4				—				1.4				—
	Foreign currency forward contracts	0.5				—				0.5				—
	_______________________________________________________________________________
	-1	Principally investments in equity securities.
	-2	Amounts are presented within Other liabilities in the accompanying Consolidated Balance Sheets.
Schedule of Changes in Level 3 Financial Assets and Liabilities	The following table presents the changes in Level 3 assets and liabilities for the years ended December 31, 2013 and 2014:
		For the Years Ended December 31,
		2013												2014
		Other Investments				Contingent Payment Arrangements				Obligations to Related Parties				Other Investments				Contingent Payment Arrangements			Obligations to Related Parties
	Balance, beginning of period	$	118.9			$	31			$	77.8			$	131.8			$	50.2		$	76.9
	Net gains/losses	12.3			(1)	19.2			(2)	6.2			(3)	11.4			(1)	9.1		(2)	5.5			(3)
	Purchases and issuances	18				—				62.9				17.4				—			96.7
	Settlements and reductions	(17.4		)		—				(70.0		)		(26.4		)		—			(86.0		)
	Net transfers in and/or out of Level 3	—				—				—				—				—			—
	Balance, end of period	$	131.8			$	50.2			$	76.9			$	134.2			$	59.3		$	93.1
	Net unrealized gains/losses relating to instruments still held at the reporting date	$	16.5			$	19.2			$	2.2			$	17.1			$	9.1		$	(0.1	)
	_______________________________________________________________________________
	-1	Gains and losses on Other investments are recorded in Investment and other income.
	-2	Accretion and changes to payment estimates under the Company's contingent payment arrangements are recorded in Imputed interest expense and contingent payment arrangements.
	-3	Gains and losses associated with agreements to repurchase Affiliate equity are recorded in Imputed interest expense and contingent payment arrangements. Gains and losses related to liabilities offsetting certain investments are recorded in Investment and other income.
Schedule of Quantitative Information	The following table presents certain quantitative information about the significant unobservable inputs used in valuing the Company's Level 3 financial liabilities:
			Quantitative Information about Level 3 Fair Value Measurements
			Valuation		Unobservable Input		Fair Value at				Range at December 31, 2013		Fair Value at				Range at December 31, 2014
			Techniques				December 31, 2013						December 31, 2014
	Contingent payment arrangements		Discounted cash flow		Growth rates		$	50.2			3% - 11%		$	59.3			6%
					Discount rates						14% - 18%						15%
	Affiliate equity repurchase obligations		Discounted cash flow		Growth rates		4				8%		21.5				5% - 9%
					Discount rates						15%						15% - 16%
Schedule of Investments	The following table summarizes, as of December 31, 2013 and 2014, the nature of these investments and any related liquidity restrictions or other factors which may impact the ultimate value realized:
		31-Dec-13								31-Dec-14
	Category of Investment	Fair Value				Unfunded				Fair Value				Unfunded
						Commitments								Commitments
	Private equity(1)	$	131.8			$	62.9			$	134.2			$	67.8
	Other funds(2)	82.3				—				75.8				—
		$	214.1			$	62.9			$	210			$	67.8
	_______________________________________________________________________________
	-1	These funds primarily invest in a broad range of private equity funds, as well as make direct investments. Distributions will be received as the underlying assets are liquidated over the life of the funds.
	-2	These are multi-disciplinary funds that invest across various asset classes and strategies including long/short equity, credit and real estate. Investments are generally redeemable on a daily or quarterly basis.
Summary of Financial Liabilities not Carried at Fair Value	he carrying amount of cash, cash equivalents, receivables, and payables and accrued liabilities approximates fair value because of the short-term nature of these instruments. The carrying value of notes receivable approximates fair value because interest rates and other terms are at market rates. The carrying value of senior bank debt approximates fair value because the debt has variable interest based on selected short-term rates. The following table summarizes the Company's other financial liabilities not carried at fair value:
		31-Dec-13								31-Dec-14
		Carrying Amount				Fair Value				Carrying Amount				Fair Value				Fair Value Hierarchy
	Senior notes	$	340			$	325			$	736.8			$	786.2			Level 2
	Convertible securities	518.7				963.9				303.1				532.1				Level 2

Business_Combinations_Tables

Business Combinations (Tables)	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Schedule of Purchase Price Allocation for Investments	The purchase price allocation for these investments is as follows:
		SouthernSun			River Road				Veritas
	Consideration paid	$	109.9		$	132.4			$	319.9
	Non-controlling interests	74			35.8				125.2
	Enterprise value	$	183.9		$	168.2			$	445.1
	Acquired client relationships	$	125.3		$	64.1			$	275.6
	Deferred income taxes	—			—				(35.7		)
	Tangible assets, net	6.3			4.5				12.6
	Goodwill	52.3			99.6				192.6
		$	183.9		$	168.2			$	445.1
Schedule of Goodwill Allocation by Segment	The excess of the enterprise value over the net assets acquired was recorded as goodwill and the segment allocation is as follows:
		SouthernSun		River Road			Veritas
	Institutional	37	%	31	%		24	%
	Mutual Fund	23	%	51	%		76	%
	High Net Worth	40	%	18	%		—	%
Schedule of Unaudited Pro Forma Financial Results	Unaudited pro-forma financial results are set forth below assuming these investments occurred on January 1, 2013, the revenue sharing arrangement had been in effect for the entire period and after making certain pro-forma adjustments.
		For the Years Ended December 31,
		2013 (Unaudited)			2014 (Unaudited)
	Revenue	$	2,353.50		$	2,644.90
	Net income (controlling interest)	382.3			474.9
	Earnings per share (basic)	$	7.2		$	8.63
	Earnings per share (diluted)	$	6.94		$	8.4

Goodwill_and_Acquired_Client_R1

Goodwill and Acquired Client Relationships (Tables)	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Changes in Goodwill	The following tables presents the change in goodwill and components of intangible assets of the Company's Affiliates that are consolidated during 2013 and 2014:
		Goodwill
		Institutional			Mutual Fund			High Net Worth			Total
	Balance, as of December 31, 2012	$	1,078.50		$	939.5		$	337.2		$	2,355.20
	Goodwill acquired	—			—			—			—
	Foreign currency translation	(2.2		)	(11.4		)	0.1			(13.5		)
	Balance, as of December 31, 2013	$	1,076.30		$	928.1		$	337.3		$	2,341.70
	Goodwill acquired	97.2			208.1			39.2			344.5
	Foreign currency translation	(14.4		)	(10.9		)	(8.1		)	(33.4		)
	Balance, as of December 31, 2014	$	1,159.10		$	1,125.30		$	368.4		$	2,652.80
Schedule of Changes in, and the Components of, Acquired Client Relationships
		Acquired Client Relationships
		Definite-lived									Indefinite-lived			Total
		Gross Book			Accumulated			Net Book			Net Book			Net Book
		Value			Amortization			Value			Value			Value
	Balance, as of December 31, 2012	$	1,109.60		$	(383.5	)	$	726.1		$	859.4		$	1,585.50
	New Investments	—			—			—			—			—
	Amortization and impairments	—			(128.2		)	(128.2		)	—			(128.2		)
	Foreign currency translation	(1.2		)	—			(1.2		)	4.6			3.4
	Transfers and other	(68.9		)	68.9			—			—			—
	Balance, as of December 31, 2013	$	1,039.50		$	(442.8	)	$	596.7		$	864		$	1,460.70
	New Investments	220.3			—			220.3			244.7			465
	Amortization and impairments	—			(122.2		)	(122.2		)	—			(122.2		)
	Foreign currency translation	(4.7		)	—			(4.7		)	(20.4		)	(25.1		)
	Transfers and other	—			—			—			—			—
	Balance, as of December 31, 2014	$	1,255.10		$	(565.0	)	$	690.1		$	1,088.30		$	1,778.40

Equity_Method_Investments_in_A1

Equity Method Investments in Affiliates (Tables)	12 Months Ended
	Dec. 31, 2014
Equity Method Investments and Joint Ventures [Abstract]
Schedule of Financial Information for Affiliates Accounted for Under the Equity Method	The following table presents summarized financial information for Affiliates accounted for under the equity method.
		For the Years Ended December 31,
		2012		2013		2014
	Revenue(1)	$	1,005.40	$	1,589.60	$	1,869.30
	Net income	783.3		1,321.90		253.8
		December 31,
		2013		2014
	Assets(2)	$	19,139.70	$	34,729.20
	Liabilities and Non-controlling interest(2)	17,763.50		33,048.60
	_______________________________________________________________________________
	-1	Revenue includes advisory fees for asset management services and net investment income from consolidated investment partnerships.
	-2	Assets consist primarily of investment securities in consolidated investment partnerships. These partnerships are generally held by investors that are unrelated to the Company.

Fixed_Assets_and_Lease_Commitm1

Fixed Assets and Lease Commitments (Tables)	12 Months Ended
	Dec. 31, 2014
Fixed Assets and Lease Commitments [Abstract]
Schedule of Fixed Assets	Fixed assets consisted of the following:
		December 31,
		2013			2014
	Building and leasehold improvements	$	78.5		$	84.1
	Equipment	36.5			38.3
	Furniture and fixtures	15.4			17.5
	Land and improvements	18.9			18.7
	Software	30.6			40.8
	Fixed assets, at cost	179.9			199.4
	Accumulated depreciation and amortization	(87.6		)	(104.0		)
	Fixed assets, net	$	92.3		$	95.4
Schedule of Aggregate Future Minimum Payments for Operating Leases	At December 31, 2014, the Company's aggregate future minimum payments for operating leases having initial or non-cancelable lease terms greater than one year are payable as follows:
		Required Minimum
		Payments
	2015	$	33.8
	2016	28.8
	2017	25.2
	2018	22.8
	2019	21.6
	Thereafter	69.4

Accounts_Payable_and_Accrued_L1

Accounts Payable and Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2014
Payables and Accruals [Abstract]
Schedule of Accounts Payable and Accrued Liabilities	Payables and accrued liabilities consisted of the following:
		December 31,
		2013		2014
	Accrued compensation and distributions	$	211.4	$	491.3
	Unsettled fund share payables	84.6		78.3
	Accrued income taxes	73		59.8
	Accrued share repurchases	—		47.8
	Accrued professional fees	33.4		26.9
	Other	112.3		104.2
		$	514.7	$	808.3

Stockholders_Equity_Tables

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Schedule of Recent Share Repurchase Activity	A summary of the Company's recent share repurchase activity is as follows:
	Period	Shares	Average
		Repurchased	Price
	2012	0.6	$	107.44
	2013	0.1	184.89
	2014	1.2	204.72

ShareBased_Compensation_Tables

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Recent Share-Based Compensation Expense	A summary of recent share-based compensation expense is as follows:
	Period		Share-Based				Tax Benefit
			Compensation
			Expense
	2012		$	25.4			$	9.6
	2013		27.5				10.6
	2014		29.3				11.3
Schedule of Transactions of the Company's Stock Options	The following table summarizes the transactions of the Company's stock options:
		Stock Options			Weighted				Weighted
					Average				Average
					Exercise				Remaining
					Price				Contractual
									Life (years)
	Unexercised options outstanding—January 1, 2014	3			$	77.71
	Options granted	0			204.99
	Options exercised	(0.7	)		60.47
	Options forfeited	(0.0	)		81.5
	Unexercised options outstanding—December 31, 2014	2.3			83.42				2.7
	Exercisable at December 31, 2014	2.1			80.92				2.6
Schedule of Assumptions Used to Determine Fair Value of Options Granted	The fair value of options granted is estimated using the Black-Scholes option pricing model. The weighted average fair value of options granted during the years ended December 31, 2012, 2013 and 2014 was $40.43, $61.82 and $60.20 per option, respectively, based on the weighted-average grant date assumptions stated below.
		For the Years Ended December 31,
		2012			2013			2014
	Dividend yield	0	%		0	%		0	%
	Expected volatility(1)	36.8	%		34.4	%		29.7	%
	Risk-free interest rate(2)	0.7	%		1.5	%		1.8	%
	Expected life of options (in years)(3)	5			5			5
	Forfeiture rate(3)	1.4	%		0	%		0	%
	_______________________________________________________________________________
	-1	Based on historical and implied volatility.
	-2	Based on the U.S. Treasury yield curve in effect at the date of grant.
	-3	Based on the Company's historical data and expected exercise behavior.
Summary of Transactions of the Company's Restricted Stock	The following table summarizes the transactions of the Company's restricted stock:
		Restricted			Weighted
		Stock			Average
					Grant Date
					Value
	Unvested units—January 1, 2014	0.5			$	176.38
	Units granted	0			190.42
	Units vested	(0.1	)		131.42
	Units forfeited	(0.0	)		173.77
	Unvested units—December 31, 2014	0.4			182.83

Affiliate_Equity_Tables

Affiliate Equity (Tables)	12 Months Ended
	Dec. 31, 2014
Affiliate Equity
Summary of Affiliate Equity Expense	A summary of Affiliate equity expense is as follows:
	Period		Affiliate Equity Expense			Tax Benefit
	2012		$	60.4		$	17
	2013		72.3			21.7
	2014		84.4			18.2
Schedule of the Changes in Redeemable Non-controlling Interests	The following table presents the changes in Redeemable non-controlling interests during the period:
		2013			2014
	Balance, as of January 1	$	477.5		$	641.9
	Transactions in Redeemable non-controlling interests	(52.5		)	(61.7		)
	Changes in redemption value	216.9			65.3
	Balance, as of December 31	$	641.9		$	645.5
Schedule of the Effect of Changes in the Company's Ownership Interest in its Affiliates on the Controlling Interest's Equity	The following schedule discloses the effect of changes in the Company's ownership interests in its Affiliates on the controlling interest's equity:
		For the Years Ended December 31,
		2012			2013			2014
	Net income (controlling interest)	$	174		$	360.5		$	452.1
	Increase (decrease) in controlling interest paid-in capital from the sale of Affiliate equity	(44.0		)	(74.0		)	(33.3		)
	Change from Net income (controlling interest) and net transfers with non-controlling interests	$	130		$	286.5		$	418.8

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Schedule of Income Tax Provision Attributable to Controlling and Non-Controlling Interests	The consolidated income tax provision includes taxes attributable to the controlling interest and, to a lesser extent, taxes attributable to non-controlling interests as follows:
		For the Years Ended December 31,
		2012				2013				2014
	Controlling Interests:
	Current tax	$	61			$	153.1			$	149.8
	Intangible-related deferred taxes	22.7				38.1				47.8
	Other deferred taxes	(12.1		)		(6.2		)		15.8
	Total controlling interests	71.6				185				213.4
	Non-controlling Interests:
	Current tax	$	11.7			$	13.3			$	15.3
	Deferred taxes	0.5				(4.2		)		(0.8		)
	Total non-controlling interests	12.2				9.1				14.5
	Provision for income taxes	$	83.8			$	194.1			$	227.9
	Income before income taxes (controlling interest)	$	245.6			$	545.5			$	665.5
	Effective tax rate attributable to controlling interests(1)	29.2		%		33.9		%		32.1		%
	_______________________________________________________________________________
	-1	Taxes attributable to the controlling interest divided by Income before income taxes (controlling interest).
Schedule of Consolidated Provision for Income Taxes	A summary of the consolidated provision for income taxes is as follows:
		For the Years Ended December 31,
		2012				2013				2014
	Current:
	Federal	$	14.5			$	104.1			$	93.8
	State	14.2				21.1				27.1
	Foreign	44				41.2				44.2
	Total current	72.7				166.4				165.1
	Deferred:
	Federal	19.8				38.1				72.5
	State	4.6				8.9				1.1
	Foreign	(13.3		)		(19.3		)		(10.8		)
	Total deferred	11.1				27.7				62.8
	Provision for income taxes	$	83.8			$	194.1			$	227.9
Schedule of Components of Income before Income Taxes	The components of income before income taxes consisted of the following:
		For the Years Ended December 31,
		2012				2013				2014
	Domestic	$	238.6			$	604			$	784.1
	International	256.6				259.7				229.5
		$	495.2			$	863.7			$	1,013.60
Schedule of Effective Income Tax Rate Computed Using Income before Income Taxes and Applying U.S. Federal Income Tax Rate	The Company's effective income tax rate differs from the amount computed by using income before income taxes and applying the U.S. federal income tax rate because of the effect of the following items:
		For the Years Ended December 31,
		2012			2013			2014
	Tax at U.S. federal income tax rate	35	%		35	%		35	%
	State income taxes, net of federal benefit(1)	3.5			2.8			2.1
	Non-deductible expenses	0.7			0.3			0.3
	Valuation allowance(1)	(4.7	)		(0.3	)		—
	Effect of Non-US operations	(3.3	)		(2.5	)		(5.3	)
	Effect of changes in tax law, rates	(2.0	)		(1.4	)		—
	Effect of income from non-controlling interests	(12.2	)		(11.4	)		(9.6	)
		17	%		22.5	%		22.5	%
Schedule of Components of Deferred Tax Assets and Liabilities	The components of deferred tax assets and liabilities are as follows:
		December 31,
		2013				2014
	Deferred Tax Assets
	State net operating loss carryforwards	$	29.3			$	15.2
	Deferred compensation	29.1				26.8
	Tax benefit of uncertain tax positions	16.2				16
	Accrued expenses	17.6				19.6
	Foreign loss carryforwards	9.1				11.5
	Other	—				1.4
	Total deferred tax assets	101.3				90.5
	Valuation allowance	(36.6		)		(18.4		)
	Deferred tax assets, net of valuation allowance	$	64.7			$	72.1
	Deferred Tax Liabilities
	Intangible asset amortization	$	(241.3	)		$	(270.9	)
	Convertible securities interest	(144.7		)		(92.5		)
	Non-deductible intangible amortization	(101.5		)		(126.4		)
	Deferred revenue	(32.1		)		(74.0		)
	Other	(2.0		)		—
	Total deferred tax liabilities	(521.6		)		(563.8		)
	Net deferred tax liability	$	(456.9	)		$	(491.7	)
Schedule of Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:
		For the Years Ended December 31,
		2012				2013				2014
	Balance, as of January 1	$	21.2			$	22.6			$	20.4
	Additions based on current year tax positions	5.9				4.1				2.6
	Additions based on prior years' tax positions	5.2				—				10.8
	Reductions for prior years' tax provisions	(5.4		)		(0.1		)		—
	Reductions related to lapses of statutes of limitations	(4.6		)		(5.4		)		(4.1		)
	Additions (reductions) related to foreign exchange rates	0.3				(0.8		)		(0.9		)
	Balance, as of December 31	$	22.6			$	20.4			$	28.8

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Schedule of Reconciliation of the Numerator and Denominator used in the Calculation of Basic and Diluted Earnings per Share	The following is a reconciliation of the numerator and denominator used in the calculation of basic and diluted earnings per share available to common stockholders.
		For the Years Ended December 31,
		2012			2013			2014
	Numerator
	Net income (controlling interest)	$	174		$	360.5		$	452.1
	Interest expense on convertible securities, net of taxes	—			10.5			15.2
	Net income (controlling interest), as adjusted	$	174		$	371		$	467.3
	Denominator
	Average shares outstanding (basic)	51.7			53.1			55
	Effect of dilutive instruments:
	Stock options and other awards	1.3			1.3			1.2
	Forward sale	—			0.3			0
	Junior convertible securities	—			2			2.2
	Average shares outstanding (diluted)	53			56.7			58.4
Diluted Earnings per Share Calculations Excluding the Anti-dilutive Effect of Shares	The diluted earnings per share calculations in the table above exclude the anti-dilutive effect of the following shares:
		For the Years Ended December 31,
		2012		2013			2014
	Stock options and other awards	0.7		0.1			0
	Senior convertible securities	3.6		2.1			—
	Junior convertible securities	4.2		2.2			0.4
	Forward equity sales	0.2		—			—

Comprehensive_Income_Tables

Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Summary of the Tax Effects Allocated to each Component of Other Comprehensive Income	The following table shows the tax effects allocated to each component of Other comprehensive income:
		For the Year Ended December 31, 2012
		Pre-Tax			Tax Benefit (Expense)			Net of Tax
	Foreign currency translation adjustment	$	18.5		$	—		$	18.5
	Change in net realized and unrealized gain (loss) on derivative securities	(1.1		)	0.4			(0.7		)
	Change in net unrealized gain (loss) on investment securities	21.6			(8.1		)	13.5
	Other comprehensive income (loss)	$	39		$	(7.7	)	$	31.3
		For the Year Ended December 31, 2013
		Pre-Tax			Tax Benefit (Expense)			Net of Tax
	Foreign currency translation adjustment	$	(19.6	)	$	—		$	(19.6	)
	Change in net realized and unrealized gain (loss) on derivative securities	1.5			(0.5		)	1
	Change in net unrealized gain (loss) on investment securities	19.5			(8.0		)	11.5
	Other comprehensive income (loss)	$	1.4		$	(8.5	)	$	(7.1	)
		For the Year Ended December 31, 2014
		Pre-Tax			Tax Benefit (Expense)			Net of Tax
	Foreign currency translation adjustment	$	(62.0	)	$	—		$	(62.0	)
	Change in net realized and unrealized gain (loss) on derivative securities	0.6			(0.3		)	0.3
	Change in net unrealized gain (loss) on investment securities	5.6			(2.2		)	3.4
	Other comprehensive income (loss)	$	(55.8	)	$	(2.5	)	$	(58.3	)
Schedule of Components of Accumulated Other Comprehensive Income, Net of Taxes	The components of Accumulated other comprehensive income, net of taxes, are as follows:
		Foreign Currency Translation Adjustment			Realized and Unrealized Losses on Derivative Securities			Unrealized Gain (Loss) on Investment Securities			Total
	Balance, as of December 31, 2013	$	56.6		$	(1.9	)	$	19.5		$	74.2
	Other comprehensive income (loss) before reclassifications	(62.0		)	0.3			1.3			(60.4		)
	Amounts reclassified from other comprehensive income	—			0			2.1			2.1
	Net other comprehensive income (loss)	(62.0		)	0.3			3.4			(58.3		)
	Balance, as of December 31, 2014	$	(5.4	)	$	(1.6	)	$	22.9		$	15.9

Selected_Quarterly_Financial_D1

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Results of Operations	The following is a summary of the quarterly results of operations of the Company for the years ended December 31, 2013 and 2014.
		2013
		First		Second		Third		Fourth
		Quarter		Quarter		Quarter		Quarter
	Revenue	$	502.2	$	541	$	551.6	$	594
	Operating income	150.5		161.4		169.1		153.1
	Income before income taxes	167.3		172.4		187.4		336.6
	Net income (controlling interest)	62.4		64.7		75.2		158.2
	Earnings per share (diluted)	$	1.15	$	1.18	$	1.37	$	2.79
		2014
		First		Second		Third		Fourth
		Quarter		Quarter (1)		Quarter (1)		Quarter
	Revenue	$	593.1	$	636.3	$	640.3	$	641.2
	Operating income	193.9		198.4		224.2		198.7
	Income before income taxes	208.2		239.1		253.4		312.8
	Net income (controlling interest)	77.2		99.1		103.2		172.6
	Earnings per share (diluted)	$	1.4	$	1.75	$	1.82	$	3.02
	_______________________________________________________________________________
	-1	In the fourth quarter of 2014, the Company finalized its purchase price allocation for its investment in EIG.  The final purchase price allocation resulted in a change in Income from equity method investments and Intangible-related deferred taxes in the second and third quarters of 2014.  Income before taxes decreased $1.6 million and $1.7 million in the second and third quarters, respectively, Net income (controlling interest) decreased $1.0 million and $1.1 million in the second and third quarters, respectively, and Earnings per share (diluted) decreased $0.02 in both periods.

Segment_Information_Tables

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Schedule of Segment Information
		As of and for the Year Ended December 31, 2012
		Institutional		Mutual Fund		High Net Worth		Total
	Revenue	$	861.3	$	774.4	$	169.8	$	1,805.50
	Net income (controlling interest)	126		27.3		20.7		174
	Total assets	3,176.50		2,354.80		655.8		6,187.10
	Goodwill	1,078.50		939.5		337.2		2,355.20
	Equity method investments in Affiliates	879.6		81.2		70.5		1,031.30
		As of and for the Year Ended December 31, 2013
		Institutional		Mutual Fund		High Net Worth		Total
	Revenue	$	948.7	$	1,023.00	$	217.1	$	2,188.80
	Net income (controlling interest)	219.9		103.4		37.2		360.5
	Total assets	3,196.50		2,448.40		673.9		6,318.80
	Goodwill	1,076.30		928.1		337.3		2,341.70
	Equity method investments in Affiliates	942.6		77.7		103		1,123.30
		As of and for the Year Ended December 31, 2014
		Institutional		Mutual Fund		High Net Worth		Total
	Revenue	$	1,022.80	$	1,242.60	$	245.5	2,510.90
	Net income (controlling interest)	227		180.1		45		452.1
	Total assets	3,739.80		3,082.00		876.3		7,698.10
	Goodwill	1,159.10		1,125.30		368.4		2,652.80
	Equity method investments in Affiliates	1,533.80		150.3		99.4		1,783.50

Business_and_Summary_of_Signif2

Business and Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2014
	Segment
	Channel
Segment Information [Abstract]
Number of operating segments	3
Number of principal distribution channels	3
Office Equipment and Furniture and Fixtures | Minimum
Fixed Assets
Estimated useful life	3 years
Office Equipment and Furniture and Fixtures | Maximum
Fixed Assets
Estimated useful life	10 years
Computer software | Minimum
Fixed Assets
Estimated useful life	3 years
Computer software | Maximum
Fixed Assets
Estimated useful life	7 years
Building
Fixed Assets
Estimated useful life	39 years

Investments_in_Marketable_Secu2

Investments in Marketable Securities (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Cost of investments in marketable securities, gross unrealized gains and losses
Total investment	$172.60	$157.90
Realized gains (losses) on available-for-sale securities	$3.40	$0

Investments_in_Marketable_Secu3

Investments in Marketable Securities - Summary of the Cost, Gross Unrealized Gains and Losses, and Fair Value of Investments (Details) (Equity Securities, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Equity Securities
Available-for-Sale
Cost	$125.60	$103.20
Unrealized Gains	42.8	33.3
Unrealized Losses	-18.1	-1.1
Fair Value	150.3	135.4
Trading
Cost	19.5	17.9
Unrealized Gains	2.9	4.6
Unrealized Losses	-0.1	0
Fair Value	$22.30	$22.50

Variable_Interest_Entities_Det

Variable Interest Entities (Details) (Sponsored Investment Funds, USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Sponsored Investment Funds
Variable Interest Entities
Unconsolidated VIE net assets	$8,550.40	$8,112.70
Carrying value and maximum exposure to loss	$1.20	$1.70

Senior_Bank_Debt_Details

Senior Bank Debt (Details) (USD $)	12 Months Ended		0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Apr. 15, 2014
Debt Instrument
Principal amount at maturity	$430,800,000	$730,800,000
Term loan outstanding	855,000,000	525,000,000
Senior Unsecured Revolving Credit Facility
Debt Instrument
Maximum borrowing capacity	1,250,000,000
Line of credit facility amount outstanding	605,000,000	525,000,000
Weighted average interest rate	1.40%	1.95%
Commitment fee amount	2,800,000	3,200,000
Senior Unsecured Term Loan
Debt Instrument
Principal amount at maturity			250,000,000
Debt instrument term			5 years
Maximum borrowing capacity, additional amount			100,000,000
Term loan outstanding	$250,000,000
Weighted average interest rate on amount outstanding	1.48%

Senior_Notes_Details

Senior Notes (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Feb. 13, 2015
	senior_note
Debt Disclosure [Abstract]
Number of senior notes outstanding	3
Debt Instrument
Principal amount at maturity	$430.80	$730.80
Subsequent Event | Senior Notes | 2025 Senior Notes
Debt Instrument
Principal amount at maturity			$350
Stated interest rate			3.50%

Senior_Notes_Summary_of_Princi

Senior Notes - Summary of Principle Terms of Senior Notes (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Debt Instrument
Par value (in millions)	$430.80	$730.80
2022 Senior Notes
Debt Instrument
Par value (in millions)	140
Stated coupon	5.25%
2024 Senior Notes
Debt Instrument
Par value (in millions)	400
Stated coupon	4.25%
2042 Senior Notes
Debt Instrument
Par value (in millions)	$200
Stated coupon	6.38%

Convertible_Securities_Details

Convertible Securities (Details) (USD $)	12 Months Ended	0 Months Ended
Share data in Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Feb. 13, 2014
Debt Instrument
Shares issued in conversion	1.9
Loss recognized on conversion of debt securities		$18,800,000
2038 Senior Convertible Notes
Debt Instrument
Repurchased face amount	460,000,000
Stated interest rate	3.95%
Repurchase amount	641,300,000
2006 Junior Convertible Securities
Debt Instrument
Outstanding principal of junior convertible trust preferred securities		300,000,000
2007 Junior Convertible Securities
Debt Instrument
Stated interest rate	5.15%
Principal amount at maturity	50
Conversion ratio (in shares)	0.25
Conversion price (in usd per share)	200
Redemption closing price trigger	260
Number of trading days closing price has exceeded threshold	20
Consecutive trading days	30 days
Deferred tax liability	8,700,000
Common Stock
Debt Instrument
Shares issued in conversion		1.9

Convertible_Securities_Schedul

Convertible Securities - Schedule of Carrying Value of Convertible Securities (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Debt Instrument
Carrying value	303.1		$518.70
Principal amount at maturity	430.8		730.8
Junior Convertible Trust Preferred Securities
Debt Instrument
Debt instrument term	23 years
2007 Junior Convertible Securities
Debt Instrument
Carrying value	303.1	[1]	301.2	[1]
Principal amount at maturity	430.8	[1]	430.8	[1]
2006 Junior Convertible Securities
Debt Instrument
Carrying value	0		217.5
Principal amount at maturity	0		$300
[1]	The carrying value is accreted to the principal amount at maturity over a remaining life of 23B years

Forward_Equity_Sale_Agreement_

Forward Equity Sale Agreement (Details) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2012
Forward Equity Sale Agreement
Notional amount sold	$77
Average share price (in dollars per share)	$185.56
Derivative remaining notional amount		252.8
Equity Contract
Forward Equity Sale Agreement
Notional amount		70.2
Amended and Restated Forward Equity Sale Agreement
Forward Equity Sale Agreement
Notional amount sold			147.2
Average share price (in dollars per share)		$198.71	$121.37
Notional amount		$400

Derivative_Financial_Instrumen2

Derivative Financial Instruments (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Interest Rate Swap
Derivatives, Fair Value
Collateral posted	$2.40
Interest Rate Swap | Other Liabilities
Derivatives, Fair Value
Fair value of derivative assets	1.4	2.5
Foreign Currency Forward Contract | Other Assets
Derivatives, Fair Value
Amount of derivative gain (loss) reported	$0.50

Derivative_Financial_Instrumen3

Derivative Financial Instruments - Schedule of Interest Rate Swap Agreements (Details) (Interest Rate Swap, USD $)	Dec. 31, 2011
In Millions, unless otherwise specified
Counterparty A Paying 1.67% Interest
Derivative
Notional amount	$25
Paying	1.67%
Counterparty A Paying 1.65% Interest
Derivative
Notional amount	25
Paying	1.65%
Counterparty B Paying 1.59% Interest
Derivative
Notional amount	25
Paying	1.59%
Counterparty B Paying 2.14% Interest
Derivative
Notional amount	$25
Paying	2.14%

Derivative_Financial_Instrumen4

Derivative Financial Instruments Derivative Financial Instruments - Schedule of Foreign Currency Contracts (Details) (Counterparty C, Foreign Currency Forward Contract)	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2014
In Millions, unless otherwise specified	Paying	Paying	Receiving	Receiving
	USD ($)	EUR (€)	USD ($)	GBP (£)
Derivative
Notional amount	$21.90	€ 28	$34.30	£ 13.5

Derivative_Financial_Instrumen5

Derivative Financial Instruments - Schedule of Derivative Instrument Gains and (Losses) before Taxes (Details) (Cash Flow Hedges, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss)
Amount of derivative gain (loss) reported	$0.60	$1.50	($1.10)
Interest Rate Swap
Derivative Instruments, Gain (Loss)
Amount of derivative gain (loss) reported	1.1	1.5	-1.1
Treasury Rate Lock
Derivative Instruments, Gain (Loss)
Amount of derivative gain (loss) reported	0	0	0
Foreign Currency Forward Contract
Derivative Instruments, Gain (Loss)
Amount of derivative gain (loss) reported	($0.50)	$0	$0

Commitments_and_Contingencies_

Commitments and Contingencies (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2015
Commitments and Contingencies
Co-investment commitments in partnership	$67.80
Acquisition agreements contingency liability	276
Expected payments to settle contingent obligations	75
Expected payments related to equity method investments	201
Net present value of expected payments to settle contingent obligations	59.3
Forecast
Commitments and Contingencies
Expected payments to settle contingent obligations		17.5
Prior Owner | Pantheon
Commitments and Contingencies
Reimbursable amount of investment commitments	$21.10

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Fair Value Disclosures [Abstract]
Significant transfers between Level I and Level II	$0

Fair_Value_Measurements_Schedu

Fair Value Measurements - Schedule of Fair Value of Financial Assets and Liabilities Measured on a Recurring Basis (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Investments in marketable securities
Other investments	$210		$214.10
Fair Value Measured on a Recurring Basis | Level 1
Financial Assets
Cash equivalents	59.1		39
Investments in marketable securities
Trading securities	22.3	[1]	22.5	[1]
Available-for-sale securities	150.3	[1]	135.4	[1]
Other investments	13.6		14.1
Financial Liabilities
Contingent payment arrangements	0	[2]	0	[2]
Obligations to related parties	0		0
Interest rate swaps	0		0
Forward currency forward contracts	0	[2]
Fair Value Measured on a Recurring Basis | Level 2
Financial Assets
Cash equivalents	0		0
Investments in marketable securities
Trading securities	0	[1]	0	[1]
Available-for-sale securities	0	[1]	0	[1]
Other investments	19.4		18.4
Financial Liabilities
Contingent payment arrangements	0	[2]	0	[2]
Obligations to related parties	0		0
Interest rate swaps	1.4		2.5
Forward currency forward contracts	0.5	[2]
Fair Value Measured on a Recurring Basis | Level 3
Financial Assets
Cash equivalents	0		0
Investments in marketable securities
Trading securities	0	[1]	0	[1]
Available-for-sale securities	0	[1]	0	[1]
Other investments	134.2		131.8
Financial Liabilities
Contingent payment arrangements	59.3	[2]	50.2	[2]
Obligations to related parties	93.1		76.9
Interest rate swaps	0		0
Forward currency forward contracts	0	[2]
Fair Value | Fair Value Measured on a Recurring Basis
Financial Assets
Cash equivalents	59.1		39
Investments in marketable securities
Trading securities	22.3	[1]	22.5	[1]
Available-for-sale securities	150.3	[1]	135.4	[1]
Other investments	167.2		164.3
Financial Liabilities
Contingent payment arrangements	59.3	[2]	50.2	[2]
Obligations to related parties	93.1		76.9
Interest rate swaps	1.4		2.5
Forward currency forward contracts	$0.50	[2]
[1]	Principally investments in equity securities.
[2]	Amounts are presented within Other liabilities in the accompanying Consolidated Balance Sheets.

Fair_Value_Measurements_Schedu1

Fair Value Measurements - Schedule of Changes in Level 3 Financial Assets and Liabilities (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Other Investments
Changes in level 3 assets and liabilities
Balance, beginning of period	$131.80		$118.90
Net gains/losses	11.4	[1]	12.3	[1]
Purchases and issuances	17.4		18
Settlements and reductions	-26.4		-17.4
Net transfers in and/or out of Level 3	0		0
Balance, end of period	134.2		131.8
Net unrealized gains/losses relating to instruments still held at the reporting date	17.1		16.5
Contingent Payment Arrangements
Changes in level 3 assets and liabilities
Balance, beginning of period	50.2		31
Net gains/losses	9.1	[2]	19.2	[2]
Purchases and issuances	0		0
Settlements and reductions	0		0
Net transfers in and/or out of Level 3	0		0
Balance, end of period	59.3		50.2
Net unrealized gains/losses relating to instruments still held at the reporting date	9.1		19.2
Obligations to Related Parties
Changes in level 3 assets and liabilities
Balance, beginning of period	76.9		77.8
Net gains/losses	5.5	[3]	6.2	[3]
Purchases and issuances	96.7		62.9
Settlements and reductions	-86		-70
Net transfers in and/or out of Level 3	0		0
Balance, end of period	93.1		76.9
Net unrealized gains/losses relating to instruments still held at the reporting date	($0.10)		$2.20
[1]	Gains and losses on Other investments are recorded in Investment and other income.
[2]	Accretion and changes to payment estimates under the Company's contingent payment arrangements are recorded in Imputed interest expense and contingent payment arrangements.
[3]	Gains and losses associated with agreements to repurchase Affiliate equity are recorded in Imputed interest expense and contingent payment arrangements. Gains and losses related to liabilities offsetting certain investments are recorded in Investment and other income.

Fair_Value_Measurements_Schedu2

Fair Value Measurements - Schedule of Quantitative Information (Details) (Discounted Cash Flow, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Contingent Payment Arrangements
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	59.3	50.2
Contingent Payment Arrangements | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	6.00%	3.00%
Discount rate (as a percent)	15.00%	14.00%
Contingent Payment Arrangements | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	6.00%	11.00%
Discount rate (as a percent)	15.00%	18.00%
Affiliate Equity Repurchase Obligations
Quantitative information for Level 3 Fair Value Measurements Liabilities
Fair value Liabilities	21.5	4
Affiliate Equity Repurchase Obligations | Minimum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	5.00%	8.00%
Discount rate (as a percent)	15.00%	15.00%
Affiliate Equity Repurchase Obligations | Maximum
Quantitative information for Level 3 Fair Value Measurements Liabilities
Growth rate (as a percent)	9.00%	8.00%
Discount rate (as a percent)	16.00%	15.00%

Fair_Value_Measurements_Schedu3

Fair Value Measurements - Schedule of Investments (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
NAV of investments at fair value
Fair value	$210		$214.10
Unfunded commitments	67.8		62.9
Private Equity Fund-of-Funds
NAV of investments at fair value
Fair value	134.2	[1]	131.8	[1]
Unfunded commitments	67.8	[1]	62.9	[1]
Other Funds
NAV of investments at fair value
Fair value	75.8	[2]	82.3	[2]
Unfunded commitments	$0	[2]	$0	[2]
[1]	These funds primarily invest in a broad range of private equity funds, as well as make direct investments. Distributions will be received as the underlying assets are liquidated over the life of the funds.
[2]	These are multi-disciplinary funds that invest across various asset classes and strategies including long/short equity, credit and real estate. Investments are generally redeemable on a daily or quarterly basis.

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Financial Liabilities not Carried at Fair Value (Details) (Level 2, USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	$786.20	$325
Convertible securities	532.1	963.9
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior notes	736.8	340
Convertible securities	$303.10	$518.70

Business_Combinations_Details

Business Combinations (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Business Acquisition
Amortization period for tax deductible goodwill	15 years
New Affiliates
Business Acquisition
New affiliate investments contributed revenue	72.2
New affiliate investments contributed earnings	8.5

Business_Combinations_Schedule

Business Combinations - Schedule of Purchase Price Allocation for Investments (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition
Goodwill	$2,652.80	$2,341.70	$2,355.20
SouthernSun
Business Acquisition
Consideration paid	109.9
Non-controlling interests	74
Enterprise value	183.9
Acquired client relationships	125.3
Deferred income taxes	0
Tangible assets, net	6.3
Goodwill	52.3
Total	183.9
River Road
Business Acquisition
Consideration paid	132.4
Non-controlling interests	35.8
Enterprise value	168.2
Acquired client relationships	64.1
Deferred income taxes	0
Tangible assets, net	4.5
Goodwill	99.6
Total	168.2
Veritas
Business Acquisition
Consideration paid	319.9
Non-controlling interests	125.2
Enterprise value	445.1
Acquired client relationships	275.6
Deferred income taxes	-35.7
Tangible assets, net	12.6
Goodwill	192.6
Total	$445.10

Business_Combinations_Schedule1

Business Combinations - Schedule of Goodwill Allocation by Segment (Details)	Dec. 31, 2014
SouthernSun | Institutional
Business Acquisition
Goodwill allocation to business segments (percent)	37.00%
SouthernSun | Mutual Fund
Business Acquisition
Goodwill allocation to business segments (percent)	23.00%
SouthernSun | High Net Worth
Business Acquisition
Goodwill allocation to business segments (percent)	40.00%
River Road | Institutional
Business Acquisition
Goodwill allocation to business segments (percent)	31.00%
River Road | Mutual Fund
Business Acquisition
Goodwill allocation to business segments (percent)	51.00%
River Road | High Net Worth
Business Acquisition
Goodwill allocation to business segments (percent)	18.00%
Veritas | Institutional
Business Acquisition
Goodwill allocation to business segments (percent)	24.00%
Veritas | Mutual Fund
Business Acquisition
Goodwill allocation to business segments (percent)	76.00%
Veritas | High Net Worth
Business Acquisition
Goodwill allocation to business segments (percent)	0.00%

Business_Combinations_Schedule2

Business Combinations - Schedule of Unaudited Pro Forma Financial Results (Details) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Business Combinations [Abstract]
Revenue	$2,644.90	$2,353.50
Net income (controlling interest)	$474.90	$382.30
Earnings per sharebbasic (in dollars per share)	$8.63	$7.20
Earnings per sharebdiluted (in dollars per share)	$8.40	$6.94

Goodwill_and_Acquired_Client_R2

Goodwill and Acquired Client Relationships (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Goodwill
Asset impairment charges	$0	$0
Acquired Client Relationships
Goodwill
Weighted average life	10 years
Future annual amortization expense	$120,000,000
Number of future annual periods of amortization expense presented	5 years

Goodwill_and_Acquired_Client_R3

Goodwill and Acquired Client Relationships - Schedule of Changes in Goodwill (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Changes in goodwill
Balance at the beginning of the period	$2,341.70	$2,355.20
Goodwill acquired	344.5	0
Foreign currency translation	-33.4	-13.5
Balance at the end of the period	2,652.80	2,341.70
Institutional
Changes in goodwill
Balance at the beginning of the period	1,076.30	1,078.50
Goodwill acquired	97.2	0
Foreign currency translation	-14.4	-2.2
Balance at the end of the period	1,159.10	1,076.30
Mutual Fund
Changes in goodwill
Balance at the beginning of the period	928.1	939.5
Goodwill acquired	208.1	0
Foreign currency translation	-10.9	-11.4
Balance at the end of the period	1,125.30	928.1
High Net Worth
Changes in goodwill
Balance at the beginning of the period	337.3	337.2
Goodwill acquired	39.2	0
Foreign currency translation	-8.1	0.1
Balance at the end of the period	$368.40	$337.30

Goodwill_and_Acquired_Client_R4

Goodwill and Acquired Client Relationships - Schedule of Changes in, and the Components of, Acquired Client Relationships (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Net Book Value
Net book value balance at the end of the period	$1,778.40	$1,460.70
Acquired Client Relationships
Definite-lived
Gross book value balance at the beginning of the period	1,039.50	1,109.60
Accumulated amortization balance at the beginning of the period	-442.8	-383.5
Net book value balance at the beginning of the period	596.7	726.1
New Investments	220.3	0
Amortization and impairments	-122.2	-128.2
Foreign currency translation	-4.7	-1.2
Transfers and other	0	-68.9
Net book value balance at the end of the period	690.1	596.7
Accumulated amortization, transfers and other	0	68.9
Net book value, transfers and other	0	0
Gross book value balance at the end of the period	1,255.10	1,039.50
Accumulated amortization balance at the end of the period	-565	-442.8
Indefinite-lived
Net book value balance at the beginning of the period	864	859.4
New investments	244.7	0
Amortization and Impairment	0	0
Foreign currency translation	-20.4	4.6
Transfers and other	0	0
Net book value balance at the end of the period	1,088.30	864
Net Book Value
Net book value balance at the beginning of the period	1,460.70	1,585.50
New investments	465	0
Amortization and Impairment	122.2	128.2
Foreign currency translation	-25.1	3.4
Total, transfers and other	0	0
Net book value balance at the end of the period	$1,778.40	$1,460.70

Equity_Method_Investments_in_A2

Equity Method Investments in Affiliates (Details) (USD $)	12 Months Ended			0 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 01, 2014	Dec. 26, 2014
Schedule of Equity Method Investments
Amortization period for tax deductible goodwill	15 years
Amortization expense during the period	$122.20	$128.20	$200
Undistributed earnings from equity method investments	133.9
Amortization expense, year 1	32
Amortization expense, year 2	32
Amortization expense, year 3	32
Amortization expense, year 4	32
Amortization expense, year 5	32
Acquired Client Relationships Under Equity Method Investments
Schedule of Equity Method Investments
Weighted average life	14 years
Amortization expense during the period	32.3	41.7
EIG Global Energy Partners, LLC | Acquired Client Relationships Under Equity Method Investments
Schedule of Equity Method Investments
Definite-lived acquired client relationships				215.2
Amortization period for tax deductible goodwill				15 years
AQR Capital Management Holdings, LLC | Acquired Client Relationships Under Equity Method Investments
Schedule of Equity Method Investments
Definite-lived acquired client relationships					122.2
Indefinite-lived acquired client relationships					$90
Amortization period for tax deductible goodwill					15 years

Equity_Method_Investments_in_A3

Equity Method Investments in Affiliates - Schedule of Financial Information for Affiliates Accounted for Under the Equity Method (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Equity Method Investments and Joint Ventures [Abstract]
Revenue	$1,869.30	[1]	$1,589.60	[1]	$1,005.40	[1]
Net income	253.8		1,321.90		783.3
Assets	34,729.20	[2]	19,139.70	[2]
Liabilities and Non-controlling interest	$33,048.60	[2]	$17,763.50	[2]
[1]	Revenue includes advisory fees for asset management services and net investment income from consolidated investment partnerships.
[2]	Assets consist primarily of investment securities in consolidated investment partnerships. These partnerships are generally held by investors that are unrelated to the Company.

Fixed_Assets_and_Lease_Commitm2

Fixed Assets and Lease Commitments (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fixed Assets and Lease Commitments [Abstract]
Consolidated rent expense	$30.50	$30.30	$30.50

Fixed_Assets_and_Lease_Commitm3

Fixed Assets and Lease Commitments - Schedule of Fixed Assets (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fixed assets
Fixed assets, at cost	$199.40	$179.90
Accumulated depreciation and amortization	-104	-87.6
Fixed assets, net	95.4	92.3
Building and Leasehold Improvements
Fixed assets
Fixed assets, at cost	84.1	78.5
Equipment
Fixed assets
Fixed assets, at cost	38.3	36.5
Furniture and Fixtures
Fixed assets
Fixed assets, at cost	17.5	15.4
Land and Improvements
Fixed assets
Fixed assets, at cost	18.7	18.9
Software
Fixed assets
Fixed assets, at cost	$40.80	$30.60

Fixed_Assets_and_Lease_Commitm4

Fixed Assets and Lease Commitments - Schedule of Aggregate Future Minimum Payments for Operating Leases (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Fixed Assets and Lease Commitments [Abstract]
2015	$33.80
2016	28.8
2017	25.2
2018	22.8
2019	21.6
Thereafter	$69.40

Accounts_Payable_and_Accrued_L2

Accounts Payable and Accrued Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Payables and Accruals [Abstract]
Accrued compensation and distributions	$491.30	$211.40
Unsettled fund share payables	78.3	84.6
Accrued income taxes	59.8	73
Accrued share repurchases	47.8	0
Accrued professional fees	26.9	33.4
Other	104.2	112.3
Accounts payable and accrued liabilities	$808.30	$514.70

Related_Party_Transactions_Det

Related Party Transactions (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Affiliate Partners
Related Party Transactions
Distributions paid to Affiliate management owners	$569.40	$267.10	$181.40
Amount paid for affiliate equity interest repurchases	32.6	36.9	8
Amount received for affiliate equity issuances	11	2.5	3.3
Affiliate Partners | Other Assets
Related Party Transactions
Receivables from related parties	20.8	21.8
Notes receivable from related parties	9.7	6.9	7.1
Affiliate Partners | Other Liabilities
Related Party Transactions
Payable to related parties	88.6	56.8
Notes payable to related parties	4.4	5.5	23.3
Prior Owner | Other Liabilities
Related Party Transactions
Investment partnerships with prior owners	71.6	72.9
Prior Owner | Non-Controlling Interests
Related Party Transactions
Investment partnerships with prior owners	$13.50	$22.20

Stockholders_Equity_Details

Stockholders' Equity (Details) (USD $)	12 Months Ended			2 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 18, 2015
Stockholders' equity
Common stock, shares authorized	153,000,000	153,000,000
Share repurchase activity
Shares repurchased (in shares)	1,200,000	100,000	600,000
Average price (in usd per share)	$204.72	$184.89	$107.44
Preferred Stock
Stockholders' equity
Preferred stock, shares authorized	5,000,000
Common Stock
Share repurchase activity
Shares repurchased (in shares)	1,200,000	100,000	600,000
Average price (in usd per share)	$204.72	$184.89	$107.44
Voting Common Stock
Stockholders' equity
Common stock, shares authorized	150,000,000
Share repurchase activity
Stock repurchase program, remaining shares authorized for repurchase	1,000,000
Class B Non-Voting Common Stock
Stockholders' equity
Common stock, shares authorized	3,000,000
Subsequent Event | Common Stock
Share repurchase activity
Shares repurchased (in shares)				300,000

ShareBased_Compensation_Stock_

Share-Based Compensation - Stock Options (Details) (USD $)	12 Months Ended			2 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 18, 2015
Share-based Compensation Arrangement by Share-based Payment Award
Cash received from stock options exercised	$41.40	$48.20
Tax benefit realized from exercise of stock options	60.2	20.7
Excess tax benefit from share-based compensation	61.5	17.3	22
Total compensation cost not yet recognized	56.8	79.3
Weighted average period for recognition (in years)	3 years
Options available for future grant under the Company's option plans (in shares)	1.7
Stock Options and Other Awards
Share-based Compensation Arrangement by Share-based Payment Award
Fair value of stock options granted	0.6	1	2.5
Options exercised	-0.7
Weighted average fair value of options granted (in usd per share)	$60.20	$61.82	$40.43
Exercises in period total intrinsic value	92.6	77.4	86.8
Intrinsic value of exercisable options	$272.90
Options available for future grant under the Company's option plans (in shares)	3.5
Stock Options and Other Awards | Minimum
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period	3 years
Expiration period following grant date	7 years
Stock Options and Other Awards | Maximum
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period	5 years
Expiration period following grant date	10 years
Subsequent Event | Stock Options and Other Awards
Share-based Compensation Arrangement by Share-based Payment Award
Options exercised				0

ShareBased_Compensation_Restri

Share-Based Compensation - Restricted Stock (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award
Weighted average period for recognition (in years)	3 years
Restricted stock available for future grant under the Company's option plans (in shares)	1.7
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award
Award fair value	$8	$34.10	$18.10
Vesting period		6 years
Market condition 1 percentage		15.00%
Market condition 2 percentage		25.00%
Market condition 3 percentage		35.00%
Expected volatility		31.60%
Risk-free interest rate		2.00%
Dividend yield		0.00%
Fair value of restricted stock awards		$19.40
Weighted average period for recognition (in years)		4 years
Restricted Stock | Minimum
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period	3 years
Restricted Stock | Maximum
Share-based Compensation Arrangement by Share-based Payment Award
Vesting period	8 years

ShareBased_Compensation_Summar

Share-Based Compensation - Summary of Recent Share-Based Compensation Expense (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based compensation expense	$29.30	$27.50	$25.40
Tax benefit	$11.30	$10.60	$9.60

ShareBased_Compensation_Schedu

Share-Based Compensation - Schedule of Transactions of the Company's Stock Options (Details) (Stock Options and Other Awards, USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014
Stock Options and Other Awards
Stock Options
Stock options outstanding at the beginning of the period (options)	3
Options granted	0
Options exercised	-0.7
Options forfeited	0
Stock options outstanding at the end of the period (options)	2.3
Exercisable stock options, at the end of the period (options)	2.1
Weighted Average Exercise Price
Stock options outstanding, at the beginning of the period (in usd per option)	$77.71
Options granted	$204.99
Options exercised	$60.47
Options forfeited	$81.50
Stock options outstanding, at the end of the period (in usd per option)	$83.42
Exercisable stock options, at the end of the period (in usd per option)	$80.92
Weighted Average Remaining Contractual Life (years)
Stock options outstanding, at the end of the period (in years)	2 years 8 months 24 days
Exercisable stock options, at the end of the period (in years)	2 years 7 months 12 days

ShareBased_Compensation_Schedu1

Share-Based Compensation - Schedule of Assumptions Used to Determine Fair Value of Options Granted (Details) (Stock Options and Other Awards)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Stock Options and Other Awards
Assumptions used to determine fair value of options granted
Dividend yield	0.00%		0.00%		0.00%
Expected volatility	29.70%	[1]	34.40%	[1]	36.80%	[1]
Risk-free interest rate	1.80%	[2]	1.50%	[2]	0.70%	[2]
Expected life of options (in years)	5 years	[3]	5 years	[3]	5 years	[3]
Forfeiture rate	0.00%	[3]	0.00%	[3]	1.40%	[3]
[1]	Based on historical and implied volatility.
[2]	Based on the U.S.B Treasury yield curve in effect at the date of grant.
[3]	Based on the Company's historical data and expected exercise behavior.

ShareBased_Compensation_Summar1

Share-Based Compensation - Summary of Transactions of the Company's Restricted Stock (Details) (Restricted Stock, USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014
Restricted Stock
Restricted Stock
Units outstanding at the beginning of the period	0.5
Units granted	0
Units vested	-0.1
Units forfeited	0
Units outstanding at the end of the period	0.4
Weighted Average Grant Date Value
Units outstanding at the beginning of the period (in dollars per share)	$176.38
Units granted	$190.42
Units vested	$131.42
Units forfeited	$173.77
Units outstanding at the end of the period (in dollars per share)	$182.83

Affiliate_Equity_Details

Affiliate Equity (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Affiliate Equity
Affiliate equity compensation expense attributable to noncontrolling interest	$37	$15.90	$16.30
Compensation expenses related to Affiliate equity	71.1	68.2
Weighted average period of recognition	4 years
Compensation expenses related to Affiliate equity, attributable to controlling interest	$41.60	$32.10

Affiliate_Equity_Summary_of_Af

Affiliate Equity - Summary of Affiliate Equity Expense (Details) (Affiliate Partners, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Affiliate Partners
Affiliate Equity
Affiliate equity expense	$84.40	$72.30	$60.40
Tax benefit	$18.20	$21.70	$17

Affiliate_Equity_Schedule_of_t

Affiliate Equity - Schedule of the Changes in Redeemable Non-controlling Interests (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Changes in redeemable non-controlling interests during the period
Beginning balance	$641.90	$477.50
Transactions in Redeemable non-controlling interests	-61.7	-52.5
Changes in redemption value	65.3	216.9
Ending balance	$645.50	$641.90

Afilliate_Equity_Schedule_of_t

Afilliate Equity - Schedule of the Effect of Changes in the Company's Ownership Interest in its Affiliates on the Controlling Interest's Equity (Details) (USD $)	3 Months Ended										12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Affiliate Equity [Abstract]
Net income (controlling interest)	$172.60	$103.20	[1]	$99.10	[1]	$77.20	$158.20	$75.20	$64.70	$62.40	$452.10	$360.50	$174
Increase (decrease) in controlling interest paid-in capital from the sale of Affiliate equity											-33.3	-74	-44
Change from Net income (controlling interest) and net transfers with non-controlling interests											$418.80	$286.50	$130
[1]	In the fourth quarter of 2014, the Company finalized its purchase price allocation for its investment in EIG. The final purchase price allocation resulted in a change in Income from equity method investments and Intangible-related deferred taxes in the second and third quarters of 2014. Income before taxes decreased $1.6 million and $1.7 million in the second and third quarters, respectively, Net income (controlling interest) decreased $1.0 million and $1.1 million in the second and third quarters, respectively, and Earnings per share (diluted) decreased $0.02 in both periods.

Benefit_Plans_Details

Benefit Plans (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	plan
Compensation and Retirement Disclosure [Abstract]
Number of defined contribution plans	3
Consolidated expenses related to benefit plans	$17.20	$14.10	$12.70

Income_Taxes_Details

Income Taxes (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Deferred tax benefit due to re-valuation of deferred taxes from a change in enacted tax rates in the United Kingdom		($11.20)	($7.30)
Deferred tax benefits from change in permanent status of foreign subsidiaries	24.5	5.9
Increase (decrease) in effective tax rate due to changes in enacted tax rate in the U.K. and deferred tax assets allowance, percentage			-6.70%
Increase (decrease) in effective tax rate due to changes in enacted tax rates in the U.K. and permanent status of foreign subsidiaries, percentage		-2.70%
Increase (decrease) in effective tax rate due to changes in deferred tax assets allowance and permanent status of foreign subsidiaries	-4.70%
Temporary difference due to repatriation of assets from a sale or liquidation of the subsidiary	139.4
Amount of deferred taxes not recognized	52.4
State net operating loss carryforwards, expiration period	18 years
Foreign operating loss carryforwards, expiration period	20 years
Liability for uncertain tax positions including interest and related charges	28.8	20.4	22.6
Accrued income tax interest and related charges	1.6	1.7	2.2
Unrecognized tax benefits that, if recognized, would affect effective tax rate	26.4	17.2	19.4
Reductions for prior years' tax provisions	0	0.1	5.4
Additions based on prior years' tax positions	$10.80	$0	$5.20

Income_Taxes_Schedule_of_Incom

Income Taxes - Schedule of Income Tax Provision Attributable to Controlling and Non-Controlling Interests (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Income tax provision
Current tax	$165.10		$166.40		$72.70
Deferred taxes	62.8		27.7		11.1
Provision for income taxes	227.9		194.1		83.8
Controlling Interests
Income tax provision
Current tax	149.8		153.1		61
Intangible-related deferred taxes	47.8		38.1		22.7
Other deferred taxes	15.8		-6.2		-12.1
Provision for income taxes	213.4		185		71.6
Income before income taxes (controlling interest)	665.5		545.5		245.6
Effective tax rate attributable to controlling interests	32.10%	[1]	33.90%	[1]	29.20%	[1]
Non-Controlling Interests
Income tax provision
Current tax	15.3		13.3		11.7
Deferred taxes	-0.8		-4.2		0.5
Provision for income taxes	$14.50		$9.10		$12.20
[1]	Taxes attributable to the controlling interest divided by Income before income taxes (controlling interest).

Income_Taxes_Schedule_of_Conso

Income Taxes - Schedule of Consolidated Provision for Income Taxes (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current:
Federal	$93.80	$104.10	$14.50
State	27.1	21.1	14.2
Foreign	44.2	41.2	44
Total current	165.1	166.4	72.7
Deferred:
Federal	72.5	38.1	19.8
State	1.1	8.9	4.6
Foreign	-10.8	-19.3	-13.3
Total deferred	62.8	27.7	11.1
Provision for income taxes	$227.90	$194.10	$83.80

Income_Taxes_Schedule_of_Compo

Income Taxes - Schedule of Components of Income before Income Taxes (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Domestic	$784.10	$604	$238.60
International	229.5	259.7	256.6
Total	$1,013.60	$863.70	$495.20

Income_Taxes_Schedule_of_Effec

Income Taxes - Schedule of Effective Income Tax Rate Computed Using Income before Income Taxes and Applying U.S. Federal Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Income Tax Disclosure [Abstract]
Tax at U.S. federal income tax rate	35.00%		35.00%		35.00%
State income taxes, net of federal benefit(1)	2.10%	[1]	2.80%	[1]	3.50%	[1]
Non-deductible expenses	0.30%		0.30%		0.70%
Valuation allowance(1)	0.00%	[1]	-0.30%	[1]	-4.70%	[1]
Effect of Non-US operations	-5.30%		-2.50%		-3.30%
Effect of changes in tax law, rates	0.00%		-1.40%		-2.00%
Effect of income from non-controlling interests	-9.60%		-11.40%		-12.20%
Total effective income tax rate	22.50%		22.50%		17.00%
[1]	State income taxes includes changes related to state valuation allowances. Valuation allowance includes changes in federal valuation allowances.

Income_Taxes_Schedule_of_Compo1

Income Taxes - Schedule of Components of Deferred Tax Assets and Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Deferred Tax Assets
State net operating loss carryforwards	$15.20	$29.30
Deferred compensation	26.8	29.1
Tax benefit of uncertain tax positions	16	16.2
Accrued expenses	19.6	17.6
Foreign loss carryforwards	11.5	9.1
Other	1.4	0
Total deferred tax assets	90.5	101.3
Valuation allowance	-18.4	-36.6
Deferred tax assets, net of valuation allowance	72.1	64.7
Deferred Tax Liabilities
Intangible asset amortization	-270.9	-241.3
Convertible securities interest	-92.5	-144.7
Non-deductible intangible amortization	-126.4	-101.5
Deferred revenue	-74	-32.1
Other	0	-2
Total deferred tax liabilities	-563.8	-521.6
Net deferred tax liability	($491.70)	($456.90)

Income_Taxes_Schedule_of_Recon

Income Taxes - Schedule of Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of beginning and ending amount of unrecognized tax benefits
Beginning balance	$20.40	$22.60	$21.20
Additions based on current year tax positions	2.6	4.1	5.9
Additions based on prior years' tax positions	10.8	0	5.2
Reductions for prior years' tax provisions	0	-0.1	-5.4
Reductions related to lapses of statutes of limitations	-4.1	-5.4	-4.6
Additions (reductions) related to foreign exchange rates	-0.9	-0.8	0.3
Ending balance	$28.80	$20.40	$22.60

Income_Taxes_Income_Taxes_Valu

Income Taxes Income Taxes - Valuation Allowance (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance due to foreign tax credit carryforwards	($6.50)	($1.60)	($11.50)
Operating Loss Carryforwards
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance due to foreign tax credit carryforwards	-15.7
Increase in State Uncertain Tax Positions
Valuation Allowance [Line Items]
Increase (decrease) in valuation allowance due to foreign tax credit carryforwards	$9.20

Earnings_Per_Share_Details

Earnings Per Share (Details) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Shares repurchased (in shares)	1.2	0.1	0.6
Average price (in usd per share)	$204.72	$184.89	$107.44

Earnings_Per_Share_Schedule_of

Earnings Per Share - Schedule of Reconciliation of the Numerator and Denominator used in the Calculation of Basic and Diluted Earnings per Share (Details) (USD $)	3 Months Ended										12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator
Net income (controlling interest)	$172.60	$103.20	[1]	$99.10	[1]	$77.20	$158.20	$75.20	$64.70	$62.40	$452.10	$360.50	$174
Interest expense on convertible securities, net of taxes											15.2	10.5	0
Net income (controlling interest), as adjusted											$467.30	$371	$174
Denominator
Average shares outstanding - basic (in shares)											55	53.1	51.7
Effect of dilutive instruments:
Stock options and other awards (in shares)											1.2	1.3	1.3
Forward sale (in shares)											0	0.3	0
Junior convertible securities (in shares)											2.2	2	0
Average shares outstanding - diluted (in shares)											58.4	56.7	53
[1]	In the fourth quarter of 2014, the Company finalized its purchase price allocation for its investment in EIG. The final purchase price allocation resulted in a change in Income from equity method investments and Intangible-related deferred taxes in the second and third quarters of 2014. Income before taxes decreased $1.6 million and $1.7 million in the second and third quarters, respectively, Net income (controlling interest) decreased $1.0 million and $1.1 million in the second and third quarters, respectively, and Earnings per share (diluted) decreased $0.02 in both periods.

Earnings_Per_Share_Diluted_Ear

Earnings Per Share - Diluted Earnings per Share Calculations Excluding the Anti-dilutive Effect of Shares (Details)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock Options and Other Awards
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings per share (in shares)	0	0.1	0.7
Senior Convertible Securities
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings per share (in shares)	0	2.1	3.6
Junior Convertible Trust Preferred Securities
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings per share (in shares)	0.4	2.2	4.2
Forward Equity Sales
Antidilutive Securities Excluded from Computation of Earnings Per Share
Antidilutive securities excluded from computation of earnings per share (in shares)	0	0	0.2

Comprehensive_Income_Summary_o

Comprehensive Income - Summary of the Tax Effects Allocated to each Component of Other Comprehensive Income (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Comprehensive Income (Loss), Pre-Tax
Foreign currency translation adjustment	($62)	($19.60)	$18.50
Change in net realized and unrealized gain (loss) on derivative securities	0.6	1.5	-1.1
Change in net unrealized gain (loss) on investment securities	5.6	19.5	21.6
Other comprehensive income (loss)	-55.8	1.4	39
Other Comprehensive Income (Loss), Tax Expenses
Change in net realized and unrealized gain (loss) on derivative securities	-0.3	-0.5	0.4
Change in net unrealized gain (loss) on investment securities	-2.2	-8	-8.1
Other comprehensive income (loss)	-2.5	-8.5	-7.7
Other Comprehensive Income (Loss), Net of Tax
Foreign currency translation adjustment	-62	-19.6	18.5
Change in net realized and unrealized gain (loss) on derivative securities, net of tax	0.3	1	-0.7
Change in net unrealized gain (loss) on investment securities	3.4	11.5	13.5
Other comprehensive income (loss)	($58.30)	($7.10)	$31.30

Comprehensive_Income_Schedule_

Comprehensive Income - Schedule of Components of Accumulated Other Comprehensive Income, Net of Taxes (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance at the beginning of the period	$74.20
Other comprehensive income (loss) before reclassifications	-60.4
Amounts reclassified from other comprehensive income	2.1
Net other comprehensive income (loss)	-58.3
Balance at the end of the period	15.9
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance at the beginning of the period	56.6
Other comprehensive income (loss) before reclassifications	-62
Amounts reclassified from other comprehensive income	0
Net other comprehensive income (loss)	-62
Balance at the end of the period	-5.4
Realized and Unrealized Losses on Derivative Securities
Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance at the beginning of the period	-1.9
Other comprehensive income (loss) before reclassifications	0.3
Amounts reclassified from other comprehensive income	0
Net other comprehensive income (loss)	0.3
Balance at the end of the period	-1.6
Unrealized Gain (Loss) on Investment Securities
Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance at the beginning of the period	19.5
Other comprehensive income (loss) before reclassifications	1.3
Amounts reclassified from other comprehensive income	2.1
Net other comprehensive income (loss)	3.4
Balance at the end of the period	$22.90

Selected_Quarterly_Financial_D2

Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Results of Operations(Details) (USD $)	3 Months Ended										12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenue	$641.20	$640.30	[1]	$636.30	[1]	$593.10	$594	$551.60	$541	$502.20	$2,510.90	$2,188.80	$1,805.50
Operating income	198.7	224.2	[1]	198.4	[1]	193.9	153.1	169.1	161.4	150.5	815.2	634.1	400.4
Income before income taxes	312.8	253.4	[1]	239.1	[1]	208.2	336.6	187.4	172.4	167.3	1,013.50	863.7	495.2
Net income (controlling interest)	172.6	103.2	[1]	99.1	[1]	77.2	158.2	75.2	64.7	62.4	452.1	360.5	174
Earnings per share - diluted (in dollars per share)	$3.02	$1.82	[1]	$1.75	[1]	$1.40	$2.79	$1.37	$1.18	$1.15	$8.01	$6.55	$3.28
EIG Global Energy Partners, LLC
Income before income taxes		1.7	[1]	1.6	[1]
Net income (controlling interest)		$1.10	[1]	$1	[1]
Earnings per share - diluted (in dollars per share)		$0.02		$0.02	[1]
[1]	In the fourth quarter of 2014, the Company finalized its purchase price allocation for its investment in EIG. The final purchase price allocation resulted in a change in Income from equity method investments and Intangible-related deferred taxes in the second and third quarters of 2014. Income before taxes decreased $1.6 million and $1.7 million in the second and third quarters, respectively, Net income (controlling interest) decreased $1.0 million and $1.1 million in the second and third quarters, respectively, and Earnings per share (diluted) decreased $0.02 in both periods.

Segment_Information_Details

Segment Information (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	Channel
	Segment
Segment Reporting [Abstract]
Number of operating segments	3
Number of principal distribution channels	3
Revenue attributable to clients domiciled outside the United States (as a percent)	37.00%	38.00%	41.00%

Segment_Information_Schedule_o

Segment Information - Schedule of Segment Information (Details) (USD $)	3 Months Ended										12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information
Revenue	$641.20	$640.30	[1]	$636.30	[1]	$593.10	$594	$551.60	$541	$502.20	$2,510.90	$2,188.80	$1,805.50
Net income (controlling interest)	172.6	103.2	[1]	99.1	[1]	77.2	158.2	75.2	64.7	62.4	452.1	360.5	174
Total assets	7,698.10						6,318.80				7,698.10	6,318.80	6,187.10
Goodwill	2,652.80						2,341.70				2,652.80	2,341.70	2,355.20
Equity method investments in Affiliates	1,031.30						1,123.30				1,031.30	1,123.30	1,783.50
Institutional
Segment Reporting Information
Revenue											1,022.80	948.7	861.3
Net income (controlling interest)											227	219.9	126
Total assets	3,739.80						3,196.50				3,739.80	3,196.50	3,176.50
Goodwill	1,159.10						1,076.30				1,159.10	1,076.30	1,078.50
Equity method investments in Affiliates	879.6						942.6				879.6	942.6	1,533.80
Mutual Fund
Segment Reporting Information
Revenue											1,242.60	1,023	774.4
Net income (controlling interest)											180.1	103.4	27.3
Total assets	3,082						2,448.40				3,082	2,448.40	2,354.80
Goodwill	1,125.30						928.1				1,125.30	928.1	939.5
Equity method investments in Affiliates	81.2						77.7				81.2	77.7	150.3
High Net Worth
Segment Reporting Information
Revenue											245.5	217.1	169.8
Net income (controlling interest)											45	37.2	20.7
Total assets	876.3						673.9				876.3	673.9	655.8
Goodwill	368.4						337.3				368.4	337.3	337.2
Equity method investments in Affiliates	$70.50						$103				$70.50	$103	$99.40
[1]	In the fourth quarter of 2014, the Company finalized its purchase price allocation for its investment in EIG. The final purchase price allocation resulted in a change in Income from equity method investments and Intangible-related deferred taxes in the second and third quarters of 2014. Income before taxes decreased $1.6 million and $1.7 million in the second and third quarters, respectively, Net income (controlling interest) decreased $1.0 million and $1.1 million in the second and third quarters, respectively, and Earnings per share (diluted) decreased $0.02 in both periods.

Schedule_II_Valuation_and_Qual1

Schedule II Valuation and Qualifying Accounts (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Income Tax Valuation Allowance
Reconciliation of beginning and ending balances of valuation and qualifying accounts
Balance at the beginning of the period	$36.60		$21.30		$35.60
Additions charged to costs and expenses	0		16.9		0
Additions charged to other accounts	0		0		0
Deductions	18.2		1.6		14.3
Balance at the end of the period	18.4		36.6		21.3
Other Allowances
Reconciliation of beginning and ending balances of valuation and qualifying accounts
Balance at the beginning of the period	8.8	[1]	8.4	[1]	9.6	[1]
Additions charged to costs and expenses	4.7	[1]	2.8	[1]	0.1	[1]
Additions charged to other accounts	0	[1]	0	[1]	0	[1]
Deductions	1.4	[1]	2.4	[1]	1.3	[1]
Balance at the end of the period	$12.10	[1]	$8.80	[1]	$8.40	[1]
[1]	Other Allowances represents reserves on notes received in connection with transfers of our interests in certain Affiliates as well as other receivable amounts, which we consider uncollectible.