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Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Results of Operations(Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Revenue | $ 641.2 us-gaap_AssetManagementFees1 | $ 640.3 us-gaap_AssetManagementFees1 | [1] | $ 636.3 us-gaap_AssetManagementFees1 | [1] | $ 593.1 us-gaap_AssetManagementFees1 | $ 594.0 us-gaap_AssetManagementFees1 | $ 551.6 us-gaap_AssetManagementFees1 | $ 541.0 us-gaap_AssetManagementFees1 | $ 502.2 us-gaap_AssetManagementFees1 | $ 2,510.9 us-gaap_AssetManagementFees1 | $ 2,188.8 us-gaap_AssetManagementFees1 | $ 1,805.5 us-gaap_AssetManagementFees1
Operating income | 198.7 us-gaap_OperatingIncomeLoss | 224.2 us-gaap_OperatingIncomeLoss | [1] | 198.4 us-gaap_OperatingIncomeLoss | [1] | 193.9 us-gaap_OperatingIncomeLoss | 153.1 us-gaap_OperatingIncomeLoss | 169.1 us-gaap_OperatingIncomeLoss | 161.4 us-gaap_OperatingIncomeLoss | 150.5 us-gaap_OperatingIncomeLoss | 815.2 us-gaap_OperatingIncomeLoss | 634.1 us-gaap_OperatingIncomeLoss | 400.4 us-gaap_OperatingIncomeLoss
Income before income taxes | 312.8 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 253.4 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | [1] | 239.1 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | [1] | 208.2 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 336.6 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 187.4 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 172.4 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 167.3 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 1,013.5 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 863.7 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest | 495.2 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Net income (controlling interest) | 172.6 us-gaap_NetIncomeLoss | 103.2 us-gaap_NetIncomeLoss | [1] | 99.1 us-gaap_NetIncomeLoss | [1] | 77.2 us-gaap_NetIncomeLoss | 158.2 us-gaap_NetIncomeLoss | 75.2 us-gaap_NetIncomeLoss | 64.7 us-gaap_NetIncomeLoss | 62.4 us-gaap_NetIncomeLoss | 452.1 us-gaap_NetIncomeLoss | 360.5 us-gaap_NetIncomeLoss | 174.0 us-gaap_NetIncomeLoss
Earnings per share - diluted (in dollars per share) | $ 3.02 us-gaap_EarningsPerShareDiluted | $ 1.82 us-gaap_EarningsPerShareDiluted | [1] | $ 1.75 us-gaap_EarningsPerShareDiluted | [1] | $ 1.40 us-gaap_EarningsPerShareDiluted | $ 2.79 us-gaap_EarningsPerShareDiluted | $ 1.37 us-gaap_EarningsPerShareDiluted | $ 1.18 us-gaap_EarningsPerShareDiluted | $ 1.15 us-gaap_EarningsPerShareDiluted | $ 8.01 us-gaap_EarningsPerShareDiluted | $ 6.55 us-gaap_EarningsPerShareDiluted | $ 3.28 us-gaap_EarningsPerShareDiluted
EIG Global Energy Partners, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Income before income taxes |  | 1.7 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember | [1] | 1.6 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember | [1] |  |  |  |  |  |  |  |
Net income (controlling interest) |  | $ 1.1 us-gaap_NetIncomeLoss / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember | [1] | $ 1.0 us-gaap_NetIncomeLoss / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember | [1] |  |  |  |  |  |  |  |
Earnings per share - diluted (in dollars per share) |  | $ 0.02 us-gaap_EarningsPerShareDiluted / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember |  | $ 0.02 us-gaap_EarningsPerShareDiluted / us-gaap_BusinessAcquisitionAxis = amg_EIGGlobalEnergyPartnersLLCMember | [1] |  |  |  |  |  |  |  |
[1] | In the fourth quarter of 2014, the Company finalized its purchase price allocation for its investment in EIG. The final purchase price allocation resulted in a change in Income from equity method investments and Intangible-related deferred taxes in the second and third quarters of 2014. Income before taxes decreased $1.6 million and $1.7 million in the second and third quarters, respectively, Net income (controlling interest) decreased $1.0 million and $1.1 million in the second and third quarters, respectively, and Earnings per share (diluted) decreased $0.02 in both periods.

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