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Income Taxes Income Taxes - Valuation Allowance (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Valuation Allowance [Line Items] |  |  |
Increase (decrease) in valuation allowance due to foreign tax credit carryforwards | $ (6.5)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount | $ (1.6)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount | $ (11.5)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
Operating Loss Carryforwards |  |  |
Valuation Allowance [Line Items] |  |  |
Increase (decrease) in valuation allowance due to foreign tax credit carryforwards | (15.7)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = us-gaap_ValuationAllowanceOperatingLossCarryforwardsMember |  |
Increase in State Uncertain Tax Positions |  |  |
Valuation Allowance [Line Items] |  |  |
Increase (decrease) in valuation allowance due to foreign tax credit carryforwards | $ 9.2 us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount / us-gaap_ValuationAllowanceByDeferredTaxAssetAxis = amg_IncreaseinStateUncertainTaxPositionsMember |  |

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- Definition Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e32123-109318 + Details
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- Details
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