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Fair Value Measurements - Schedule of Changes in Level 3 Financial Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Other Investments |  |  |  |
Changes in level 3 assets and liabilities |  |  |  |
Balance, beginning of period | $ 131.8 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |  | $ 118.9 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |
Net gains/losses | 11.4 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember | [1] | 12.3 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember | [1]
Purchases and issuances | 17.4 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |  | 18.0 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |
Settlements and reductions | (26.4)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |  | (17.4)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |
Net transfers in and/or out of Level 3 | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |
Balance, end of period | 134.2 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |  | 131.8 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |
Net unrealized gains/losses relating to instruments still held at the reporting date | 17.1 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |  | 16.5 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = us-gaap_OtherInvestmentsMember |
Contingent Payment Arrangements |  |  |  |
Changes in level 3 assets and liabilities |  |  |  |
Balance, beginning of period | 50.2 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |  | 31.0 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |
Net gains/losses | 9.1 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember | [2] | 19.2 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember | [2]
Purchases and issuances | 0 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |  | 0 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |
Settlements and reductions | 0 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |  | 0 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |
Net transfers in and/or out of Level 3 | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |
Balance, end of period | 59.3 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |  | 50.2 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |
Net unrealized gains/losses relating to instruments still held at the reporting date | 9.1 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |  | 19.2 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = amg_ContingentPaymentArrangementsMember |
Obligations to Related Parties |  |  |  |
Changes in level 3 assets and liabilities |  |  |  |
Balance, beginning of period | 76.9 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |  | 77.8 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |
Net gains/losses | 5.5 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember | [3] | 6.2 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember | [3]
Purchases and issuances | 96.7 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |  | 62.9 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |
Settlements and reductions | (86.0)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |  | (70.0)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |
Net transfers in and/or out of Level 3 | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |
Balance, end of period | 93.1 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |  | 76.9 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |
Net unrealized gains/losses relating to instruments still held at the reporting date | $ (0.1)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |  | $ 2.2 amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings / amg_FairValueByAssetAndLiabilityClassAxis = amg_ObligationsToRelatedPartiesMember |
[1] | Gains and losses on Other investments are recorded in Investment and other income.
[2] | Accretion and changes to payment estimates under the Company's contingent payment arrangements are recorded in Imputed interest expense and contingent payment arrangements.
[3] | Gains and losses associated with agreements to repurchase Affiliate equity are recorded in Imputed interest expense and contingent payment arrangements. Gains and losses related to liabilities offsetting certain investments are recorded in Investment and other income.

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- Definition Represents purchases, sales, issuances, and settlements (net) which have taken place during the period in relation to assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3). + References No definition available. + Details
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- Definition Represents total unrealized gains or losses for the period, arising from assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or other comprehensive income. + References No definition available. + Details
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- Definition Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy. + References No definition available. + Details
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