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Senior Notes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 senior_note	Dec. 31, 2013	Feb. 13, 2015
Debt Disclosure [Abstract]
Number of senior notes outstanding	3amg_NumberOfDebtInstrumentsOutstanding
Debt Instrument
Principal amount at maturity	$ 430.8us-gaap_DebtInstrumentFaceAmount	$ 730.8us-gaap_DebtInstrumentFaceAmount
Subsequent Event | Senior Notes | 2025 Senior Notes
Debt Instrument
Principal amount at maturity			$ 350.0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_SeniorNotesThreePointFiveZeroPercentDue2025Member / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Stated interest rate			3.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_SeniorNotesThreePointFiveZeroPercentDue2025Member / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

X

- Definition

Represents the number of debt instruments outstanding during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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- Details

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

X

- 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

- 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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Balance Type:	na
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- Details

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- Details

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