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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	303.1amg_JuniorConvertibleTrustPreferredDebtNoncurrent		$ 518.7amg_JuniorConvertibleTrustPreferredDebtNoncurrent
Principal amount at maturity	430.8us-gaap_DebtInstrumentFaceAmount		730.8us-gaap_DebtInstrumentFaceAmount
Junior Convertible Trust Preferred Securities
Debt Instrument
Debt instrument term	23 years
2007 Junior Convertible Securities
Debt Instrument
Carrying value	303.1amg_JuniorConvertibleTrustPreferredDebtNoncurrent / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member	[1]	301.2amg_JuniorConvertibleTrustPreferredDebtNoncurrent / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member	[1]
Principal amount at maturity	430.8us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member	[1]	430.8us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2007Member	[1]
2006 Junior Convertible Securities
Debt Instrument
Carrying value	0amg_JuniorConvertibleTrustPreferredDebtNoncurrent / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2006Member		217.5amg_JuniorConvertibleTrustPreferredDebtNoncurrent / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2006Member
Principal amount at maturity	0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_JuniorConvertibleTrustPreferredSecurities2006Member		$ 300.0us-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

X

- 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

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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

X

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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

X

- Details

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X

- Details

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Data Type:	na
Balance Type:
Period Type:

X

- Details

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