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Equity Method Investments in Affiliates (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 0 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Apr. 01, 2014 | Dec. 26, 2014
Schedule of Equity Method Investments |  |  |  |  |
Amortization period for tax deductible goodwill | 15 years |  |  |  |
Amortization expense during the period | $ 122.2 us-gaap_AmortizationOfIntangibleAssets | $ 128.2 us-gaap_AmortizationOfIntangibleAssets | $ 200.0 us-gaap_AmortizationOfIntangibleAssets |  |
Undistributed earnings from equity method investments | 133.9 us-gaap_RetainedEarningsUndistributedEarningsFromEquityMethodInvestees |  |  |  |
Amortization expense, year 1 | 32.0 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear |  |  |  |
Amortization expense, year 2 | 32.0 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo |  |  |  |
Amortization expense, year 3 | 32.0 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree |  |  |  |
Amortization expense, year 4 | 32.0 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour |  |  |  |
Amortization expense, year 5 | 32.0 us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFive |  |  |  |
Acquired Client Relationships Under Equity Method Investments |  |  |  |  |
Schedule of Equity Method Investments |  |  |  |  |
Weighted average life | 14 years |  |  |  |
Amortization expense during the period | 32.3 us-gaap_AmortizationOfIntangibleAssets / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember | 41.7 us-gaap_AmortizationOfIntangibleAssets / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember |  |  |
EIG Global Energy Partners, LLC | Acquired Client Relationships Under Equity Method Investments |  |  |  |  |
Schedule of Equity Method Investments |  |  |  |  |
Definite-lived acquired client relationships |  |  |  | 215.2 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember |
Amortization period for tax deductible goodwill |  |  |  | 15 years |
AQR Capital Management Holdings, LLC | Acquired Client Relationships Under Equity Method Investments |  |  |  |  |
Schedule of Equity Method Investments |  |  |  |  |
Definite-lived acquired client relationships |  |  |  |  | 122.2 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles / us-gaap_BusinessAcquisitionAxis = amg_AQRCapitalManagementHoldingsLLCMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember
Indefinite-lived acquired client relationships |  |  |  |  | $ 90.0 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets / us-gaap_BusinessAcquisitionAxis = amg_AQRCapitalManagementHoldingsLLCMember / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = amg_AcquiredClientRelationshipsOfEquityMethodInvestmentsMember
Amortization period for tax deductible goodwill |  |  |  |  | 15 years

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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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- Definition Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472 + Details
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- Definition Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized in the remainder of the fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
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- Definition Amount of amortization expense for assets, excluding financial assets and goodwill, lacking physical substance with a finite life expected to be recognized during the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275 + Details
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