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Convertible Securities (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	12 Months Ended	0 Months Ended
	Dec. 31, 2014	Feb. 13, 2014
Debt Instrument
Shares issued in conversion	1.9us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities
Loss recognized on conversion of debt securities		$ 18,800,000amg_GainLossonConversionofDebtSecurities
2038 Senior Convertible Notes
Debt Instrument
Repurchased face amount	460,000,000us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = amg_SeniorConvertibleNotesThreePointNineFivePercentDue2038Member
Stated interest rate	3.95%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_SeniorConvertibleNotesThreePointNineFivePercentDue2038Member
Repurchase amount	641,300,000us-gaap_DebtInstrumentRepurchaseAmount / us-gaap_DebtInstrumentAxis = amg_SeniorConvertibleNotesThreePointNineFivePercentDue2038Member
2006 Junior Convertible Securities
Debt Instrument
Outstanding principal of junior convertible trust preferred securities		300,000,000us-gaap_DebtConversionOriginalDebtAmount1 / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2006Member
2007 Junior Convertible Securities
Debt Instrument
Stated interest rate	5.15%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member
Principal amount at maturity	50amg_DebtInstrumentIndividualSecurityFaceAmount / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member
Conversion ratio (in shares)	0.25us-gaap_DebtInstrumentConvertibleConversionRatio1 / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member
Conversion price (in usd per share)	200us-gaap_DebtInstrumentConvertibleConversionPrice1 / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member
Redemption closing price trigger	260us-gaap_DebtInstrumentConvertibleStockPriceTrigger / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member
Number of trading days closing price has exceeded threshold	20us-gaap_DebtInstrumentConvertibleThresholdTradingDays / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member
Consecutive trading days	30 days
Deferred tax liability	8,700,000amg_DeferredTaxLiabilityFromExcessInterestExpenseCalculation / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member
Common Stock
Debt Instrument
Shares issued in conversion		1.9us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember

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Debt Instrument, Individual Security, Face Amount

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Gain (Loss) on Conversion of Debt Securities

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- Definition

The amount of the original debt being converted in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 5

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The price per share of the conversion feature embedded in the debt instrument.

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Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount.

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Price of the entity's common stock which would be required to be attained for the conversion feature embedded in the debt instrument to become effective.

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Threshold period of specified consecutive trading days within which common stock price to conversion price of convertible debt instruments must exceed threshold percentage for a specified number of trading days to trigger conversion feature, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Threshold number of specified trading days that common stock price to conversion price of convertible debt instruments must exceed threshold percentage within a specified consecutive trading period to trigger conversion feature.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Fair value amount of debt instrument that was repurchased.

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Face (par) amount of the original debt instrument that was repurchased.

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Number of shares issued during the period as a result of the conversion of convertible securities.

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