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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 | 1.9 us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities |
Loss recognized on conversion of debt securities |  | $ 18,800,000 amg_GainLossonConversionofDebtSecurities
2038 Senior Convertible Notes |  |
Debt Instrument |  |
Repurchased face amount | 460,000,000 us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_DebtInstrumentAxis = amg_SeniorConvertibleNotesThreePointNineFivePercentDue2038Member |
Stated interest rate | 3.95%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_SeniorConvertibleNotesThreePointNineFivePercentDue2038Member |
Repurchase amount | 641,300,000 us-gaap_DebtInstrumentRepurchaseAmount / us-gaap_DebtInstrumentAxis = amg_SeniorConvertibleNotesThreePointNineFivePercentDue2038Member |
2006 Junior Convertible Securities |  |
Debt Instrument |  |
Outstanding principal of junior convertible trust preferred securities |  | 300,000,000 us-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 | 50 amg_DebtInstrumentIndividualSecurityFaceAmount / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member |
Conversion ratio (in shares) | 0.25 us-gaap_DebtInstrumentConvertibleConversionRatio1 / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member |
Conversion price (in usd per share) | 200 us-gaap_DebtInstrumentConvertibleConversionPrice1 / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member |
Redemption closing price trigger | 260 us-gaap_DebtInstrumentConvertibleStockPriceTrigger / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member |
Number of trading days closing price has exceeded threshold | 20 us-gaap_DebtInstrumentConvertibleThresholdTradingDays / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member |
Consecutive trading days | 30 days |
Deferred tax liability | 8,700,000 amg_DeferredTaxLiabilityFromExcessInterestExpenseCalculation / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member |
Common Stock |  |
Debt Instrument |  |
Shares issued in conversion |  | 1.9 us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember

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- Definition Represents the deferred taxes generated from the excess of the interest expense calculated under federal income tax regulations over interest expense as calculated for book purposes. + References No definition available. + Details
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- Definition Gain (Loss) on Conversion of Debt Securities + References No definition available. + Details
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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 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + Details
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- Definition The price per share of the conversion feature embedded in the debt instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6928298&loc=SL6031898-161870 + Details
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- Definition Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(5)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Conversion Rate -URI http://asc.fasb.org/extlink&oid=6509012 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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