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Comprehensive Income - Schedule of Components of Accumulated Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015
Total |
Components of Accumulated other comprehensive income, net of taxes |
Balance at the beginning of the period (in dollars) | $ 15.9 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
Other comprehensive income (loss) before reclassifications | (49.1)amg_OtherComprehensiveIncomeLossBeforeReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
Amounts reclassified from other comprehensive income | 0.4 amg_OtherComprehensiveIncomeLossReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
Net other comprehensive income (loss) | (48.7)amg_OtherComprehensiveIncomeLossAfterReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
Balance at the end of the period (in dollars) | (32.8)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
Foreign Currency Translation Adjustment |
Components of Accumulated other comprehensive income, net of taxes |
Balance at the beginning of the period (in dollars) | (5.4)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Other comprehensive income (loss) before reclassifications | (60.1)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) | (60.1)amg_OtherComprehensiveIncomeLossAfterReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Balance at the end of the period (in dollars) | (65.5)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember
Realized and Unrealized Losses on Derivative Securities |
Components of Accumulated other comprehensive income, net of taxes |
Balance at the beginning of the period (in dollars) | (1.6)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Other comprehensive income (loss) before reclassifications | 2.4 amg_OtherComprehensiveIncomeLossBeforeReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Amounts reclassified from other comprehensive income | 0.3 amg_OtherComprehensiveIncomeLossReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Net other comprehensive income (loss) | 2.7 amg_OtherComprehensiveIncomeLossAfterReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Balance at the end of the period (in dollars) | 1.1 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Unrealized Gain (Loss) on Investment Securities |
Components of Accumulated other comprehensive income, net of taxes |
Balance at the beginning of the period (in dollars) | 22.9 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Other comprehensive income (loss) before reclassifications | 8.6 amg_OtherComprehensiveIncomeLossBeforeReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Amounts reclassified from other comprehensive income | 0.1 amg_OtherComprehensiveIncomeLossReclassificationNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Net other comprehensive income (loss) | 8.7 amg_OtherComprehensiveIncomeLossAfterReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Balance at the end of the period (in dollars) | $ 31.6 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember

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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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