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Income Taxes (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Income Tax Provision |  |  |  |
Undistributed Earnings of Foreign Subsidiaries | 6.3 us-gaap_UndistributedEarningsOfForeignSubsidiaries |  |  |
Liability for uncertain tax positions including interest and related charges | 27.7 amg_UnrecognizedTaxBenefitsIncludingTaxInterestAndRelatedChargesAccrued |  |  |
Accrued income tax interest and related charges | 1.8 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued |  |  |
Unrecognized tax benefits that, if recognized, would affect effective tax rate | 25.5 us-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).

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- 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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- 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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- 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 -Subparagraph (b) -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 + Details
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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 + Details
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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 + Details
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