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Senior Notes - Summary of Principle Terms of Senior Notes (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument
Par value (in millions)	$ 430.8us-gaap_DebtInstrumentFaceAmount	$ 730.8us-gaap_DebtInstrumentFaceAmount
2022 Senior Notes
Debt Instrument
Par value (in millions)	140.0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_SeniorNotesFivePointTwoFivePercentDue2022Member
Stated coupon	5.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_SeniorNotesFivePointTwoFivePercentDue2022Member
2024 Senior Notes
Debt Instrument
Par value (in millions)	400.0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_SeniorNotesFivePointTwoFivePercentDue2024Member
Stated coupon	4.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_SeniorNotesFivePointTwoFivePercentDue2024Member
2042 Senior Notes
Debt Instrument
Par value (in millions)	$ 200.0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = amg_SeniorNotesSixPointThreeSevenFivePercentDue2042Member
Stated coupon	6.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = amg_SeniorNotesSixPointThreeSevenFivePercentDue2042Member

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

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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

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