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Comprehensive Income - Schedule of Components of Accumulated Other Comprehensive Income, Net of Taxes (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward] |
Balance at the beginning of the period | $ 74.2 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Other comprehensive income (loss) before reclassifications | (60.4)amg_OtherComprehensiveIncomeLossBeforeReclassificationNetOfTax
Amounts reclassified from other comprehensive income | 2.1 amg_OtherComprehensiveIncomeLossReclassificationNetOfTax
Net other comprehensive income (loss) | (58.3)amg_OtherComprehensiveIncomeLossAfterReclassificationsNetOfTax
Balance at the end of the period | 15.9 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Foreign Currency Translation Adjustment |
Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward] |
Balance at the beginning of the period | 56.6 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Other comprehensive income (loss) before reclassifications | (62.0)amg_OtherComprehensiveIncomeLossBeforeReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Amounts reclassified from other comprehensive income | 0 amg_OtherComprehensiveIncomeLossReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Net other comprehensive income (loss) | (62.0)amg_OtherComprehensiveIncomeLossAfterReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Balance at the end of the period | (5.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Realized and Unrealized Losses on Derivative Securities |
Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward] |
Balance at the beginning of the period | (1.9)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Other comprehensive income (loss) before reclassifications | 0.3 amg_OtherComprehensiveIncomeLossBeforeReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Amounts reclassified from other comprehensive income | 0 amg_OtherComprehensiveIncomeLossReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Net other comprehensive income (loss) | 0.3 amg_OtherComprehensiveIncomeLossAfterReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Balance at the end of the period | (1.6)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Unrealized Gain (Loss) on Investment Securities |
Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward] |
Balance at the beginning of the period | 19.5 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Other comprehensive income (loss) before reclassifications | 1.3 amg_OtherComprehensiveIncomeLossBeforeReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Amounts reclassified from other comprehensive income | 2.1 amg_OtherComprehensiveIncomeLossReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Net other comprehensive income (loss) | 3.4 amg_OtherComprehensiveIncomeLossAfterReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Balance at the end of the period | $ 22.9 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember

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- Definition Represents the net of tax amount of other comprehensive income after reclassifications. + References No definition available. + Details
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Period Type: | duration

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- Definition Represents the net of tax amount of other comprehensive income before reclassifications. + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Represents the net of tax amount of the amounts reclassified from other comprehensive income. + References No definition available. + Details
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- Definition Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e681-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 + Details
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