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Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Abstract]      
Deferred tax benefit due to re-valuation of deferred taxes from a change in enacted tax rates in the United Kingdom   $ (11.2)us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate $ (7.3)us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate
Deferred tax benefits from change in permanent status of foreign subsidiaries 24.5amg_EffectiveIncomeTaxRateReconciliationChangeinPermanentStatusofForeignSubsidiariesAmount 5.9amg_EffectiveIncomeTaxRateReconciliationChangeinPermanentStatusofForeignSubsidiariesAmount  
Increase (decrease) in effective tax rate due to changes in enacted tax rate in the U.K. and deferred tax assets allowance, percentage     (6.70%)amg_EffectiveIncomeTaxRateReconciliationChangeinEnactedTaxRateandChangeinDeferredTaxAssetsAllowancePercent
Increase (decrease) in effective tax rate due to changes in enacted tax rates in the U.K. and permanent status of foreign subsidiaries, percentage   (2.70%)amg_EffectiveIncomeTaxRateReconciliationChangeinEnactedTaxRateandChangeinPermanentStatusofForeignSubsidiariesPercent  
Increase (decrease) in effective tax rate due to changes in deferred tax assets allowance and permanent status of foreign subsidiaries (4.70%)amg_EffectiveIncomeTaxRateReconciliationChangeinPermanentStatusofForeignSubsidiariesandChangeinDeferredAssetsValuationAllowancePercent    
Temporary difference due to repatriation of assets from a sale or liquidation of the subsidiary 139.4amg_AmountOfTemporaryDifferenceDueToRepatriationOfAssetsFromSaleOrLiquidationOfSubsidiary    
Amount of deferred taxes not recognized 52.4amg_DeferredTaxNotRecognizedAmountOfTemporaryDifference    
State net operating loss carryforwards, expiration period 18 years    
Foreign operating loss carryforwards, expiration period 20 years    
Liability for uncertain tax positions including interest and related charges 28.8amg_UnrecognizedTaxBenefitsIncludingTaxInterestAndRelatedChargesAccrued 20.4amg_UnrecognizedTaxBenefitsIncludingTaxInterestAndRelatedChargesAccrued 22.6amg_UnrecognizedTaxBenefitsIncludingTaxInterestAndRelatedChargesAccrued
Accrued income tax interest and related charges 1.6us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 1.7us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 2.2us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued
Unrecognized tax benefits that, if recognized, would affect effective tax rate 26.4us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 17.2us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 19.4us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Reductions for prior years' tax provisions 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions 0.1us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions 5.4us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
Additions based on prior years' tax positions $ 10.8us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions $ 0us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions $ 5.2us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions