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Fair Value Measurements - Schedule of Investments (Details) (USD $)
In Millions, unless otherwise specified

Dec. 31, 2014

Dec. 31, 2013

NAV of investments at fair value

Fair value

$ 210.0us-gaap_AlternativeInvestmentsFairValueDisclosure

$ 214.1us-gaap_AlternativeInvestmentsFairValueDisclosure

Unfunded commitments

67.8us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments

62.9us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments

Private Equity Fund-of-Funds

NAV of investments at fair value

Fair value

134.2us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember

^[1]

131.8us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember

^[1]

Unfunded commitments

67.8us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember

^[1]

62.9us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember

^[1]

Other Funds

NAV of investments at fair value

Fair value

75.8us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= amg_OtherFundsMember

^[2]

82.3us-gaap_AlternativeInvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= amg_OtherFundsMember

^[2]

Unfunded commitments

$ 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= amg_OtherFundsMember

^[2]

$ 0us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments
/ us-gaap_InvestmentTypeAxis
= amg_OtherFundsMember

^[2]

[1]	These funds primarily invest in a broad range of private equity funds, as well as make direct investments. Distributions will be received as the underlying assets are liquidated over the life of the funds.
[2]	These are multi-disciplinary funds that invest across various asset classes and strategies including long/short equity, credit and real estate. Investments are generally redeemable on a daily or quarterly basis.

X

- Definition

Fair value of investments in certain entities that calculate net asset value per share, including, but not limited to, by unit, membership interest, or other ownership interest. Investments include, but are not limited to, investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258

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X

- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 6A

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258

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X

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X

- Details

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