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Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Income Tax Provision
Undistributed Earnings of Foreign Subsidiaries	6.3us-gaap_UndistributedEarningsOfForeignSubsidiaries
Liability for uncertain tax positions including interest and related charges	27.7amg_UnrecognizedTaxBenefitsIncludingTaxInterestAndRelatedChargesAccrued
Accrued income tax interest and related charges	1.8us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued
Unrecognized tax benefits that, if recognized, would affect effective tax rate	25.5us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Parent Company
Income Tax Provision
Effective tax rate attributable to controlling interests	34.10%amg_IncomeTaxExpenseFromControllingInterestToPretaxIncomeFromContinuedOperationsAttributableToControllingInterestRatio / dei_LegalEntityAxis = us-gaap_ParentCompanyMember	[1]	37.10%amg_IncomeTaxExpenseFromControllingInterestToPretaxIncomeFromContinuedOperationsAttributableToControllingInterestRatio / dei_LegalEntityAxis = us-gaap_ParentCompanyMember	[1]
[1] Taxes attributable to the controlling interest divided by Income before income taxes (controlling interest).

X

- Definition

A ratio calculated by dividing the amount of income tax expense attributable to the controlling interest by pretax income from continuing operations attributable to the controlling interest.

+ References

No definition available.

+ Details

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X

- Definition

The gross amount of unrecognized tax benefits, including interest and related charges, pertaining to uncertain tax positions taken in tax returns as of the balance sheet date.

+ References

No definition available.

+ Details

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X

- Definition

Amount of undistributed earnings of foreign subsidiaries intended to be permanently reinvested outside the country of domicile.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (b)(c)

-URI http://asc.fasb.org/extlink&oid=6424122&loc=d3e41874-109331



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 740

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6479915&loc=d3e66715-112838



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 740

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6487024&loc=d3e29054-158556



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 995

-SubTopic 740

-Section 50

-Paragraph 2

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-URI http://asc.fasb.org/extlink&oid=6503627&loc=d3e59836-110443



Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 25

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28200-109314

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- Definition

Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319

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- Definition

The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319

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