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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

Dec. 31, 2014

Dec. 31, 2013

Other Investments

Changes in level 3 assets and liabilities

Balance, beginning of period

$ 131.8amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

$ 118.9amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

Net gains/losses

11.4amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

^[1]

12.3amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

^[1]

Purchases and issuances

17.4amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

18.0amg_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

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

Balance, end of period

134.2amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

131.8amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

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

17.1amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

16.5amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= us-gaap_OtherInvestmentsMember

Contingent Payment Arrangements

Changes in level 3 assets and liabilities

Balance, beginning of period

50.2amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

31.0amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

Net gains/losses

9.1amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

^[2]

19.2amg_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

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

Balance, end of period

59.3amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

50.2amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

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

9.1amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

19.2amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisUnrealizedAssetAndLiabilitiesGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ContingentPaymentArrangementsMember

Obligations to Related Parties

Changes in level 3 assets and liabilities

Balance, beginning of period

76.9amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

77.8amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

Net gains/losses

5.5amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

^[3]

6.2amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesRealizedGainLossIncludedInEarnings
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

^[3]

Purchases and issuances

96.7amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesPurchasesSalesIssuancesSettlements
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

62.9amg_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

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

Balance, end of period

93.1amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAndLiabilitiesValue
/ amg_FairValueByAssetAndLiabilityClassAxis
= amg_ObligationsToRelatedPartiesMember

76.9amg_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

$ 2.2amg_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 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).

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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

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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.

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