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Fair Value Measurements - Schedule of Changes in Level 3 Assets and Liabilities (Details) (USD $)
In Millions, unless otherwise specified

3 Months Ended

Mar. 31, 2015

Mar. 31, 2014

Other Investments

Changes in level 3 assets and liabilities

Balance, beginning of period

$ 134.2amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

$ 131.8amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

Net gains/losses

(4.2)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

^[1]

5.8amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

^[1]

Purchases and issuances

2.5amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

3.7amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

Settlements and reductions

(5.2)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

(5.4)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

Net transfers in and/or out of Level 3

6.5amg_FairvalueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetsandLiabilitiesTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

0amg_FairvalueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetsandLiabilitiesTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

Balance, end of period

133.8amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

135.9amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

Net unrealized gains/losses relating to instruments still held at the reporting date

(1.2)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

^[1]

6.9amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

^[1]

Contingent Payment Arrangements

Changes in level 3 assets and liabilities

Balance, beginning of period

(59.3)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

(50.2)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

Net gains/losses

27.8amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

^[2]

(3.0)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

^[2]

Purchases and issuances

0amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

0amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

Settlements and reductions

0amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

0amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

Net transfers in and/or out of Level 3

0amg_FairvalueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetsandLiabilitiesTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

0amg_FairvalueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetsandLiabilitiesTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

Balance, end of period

(31.5)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

(53.2)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

Net unrealized gains/losses relating to instruments still held at the reporting date

27.8amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

^[2]

(3.0)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

^[2]

Obligations To Related Parties

Changes in level 3 assets and liabilities

Balance, beginning of period

(93.1)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

(76.9)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

Net gains/losses

0.1amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

^[3]

(2.5)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

^[3]

Purchases and issuances

(16.8)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

(59.4)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

Settlements and reductions

23.7amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

6.9amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlementsAndReductions
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

Net transfers in and/or out of Level 3

0amg_FairvalueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetsandLiabilitiesTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

0amg_FairvalueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetsandLiabilitiesTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

Balance, end of period

(86.1)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

(131.9)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

Net unrealized gains/losses relating to instruments still held at the reporting date

$ 4.0amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

^[3]

$ (0.9)amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

^[3]

[1]	Gains and losses on Other investments are recorded in Investment and other income.
[2]	Accretion and changes to 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 total realized 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.

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- Definition

Represents assets and liabilities measured at fair value using significant unobservable inputs (Level 3) which are required for reconciliation purposes of beginning and ending balances.

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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.

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- Definition

Represents settlements and reductions 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.

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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.

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