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Convertible Securities - Schedule of Carrying Value of Convertible Securities (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013
Debt Instrument |  |  |  |
Carrying value | 303.1 amg_JuniorConvertibleTrustPreferredDebtNoncurrent |  | $ 518.7 amg_JuniorConvertibleTrustPreferredDebtNoncurrent |
Principal amount at maturity | 430.8 us-gaap_DebtInstrumentFaceAmount |  | 730.8 us-gaap_DebtInstrumentFaceAmount |
Junior Convertible Trust Preferred Securities |  |  |  |
Debt Instrument |  |  |  |
Debt instrument term | 23 years |  |  |
2007 Junior Convertible Securities |  |  |  |
Debt Instrument |  |  |  |
Carrying value | 303.1 amg_JuniorConvertibleTrustPreferredDebtNoncurrent / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member | [1] | 301.2 amg_JuniorConvertibleTrustPreferredDebtNoncurrent / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member | [1]
Principal amount at maturity | 430.8 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member | [1] | 430.8 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member | [1]
2006 Junior Convertible Securities |  |  |  |
Debt Instrument |  |  |  |
Carrying value | 0 amg_JuniorConvertibleTrustPreferredDebtNoncurrent / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2006Member |  | 217.5 amg_JuniorConvertibleTrustPreferredDebtNoncurrent / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2006Member |
Principal amount at maturity | 0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2006Member |  | $ 300.0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2006Member |
[1] | The carrying value is accreted to the principal amount at maturity over a remaining life of 23 years

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- Definition Carrying amount of junior long-term convertible debt as of the balance sheet date, net of the amount due in the next twelve months. The debt is convertible, at any time, into a defined number of shares of common stock. Holders of the junior convertible trust preferred securities have no rights to put these securities. + References No definition available. + Details
Name: | amg_JuniorConvertibleTrustPreferredDebtNoncurrent
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
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Data Type: | na
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- Details
Name: | us-gaap_DebtInstrumentAxis=amg_JuniorConvertibleTrustPreferredSecurities2007Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=amg_JuniorConvertibleTrustPreferredSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |