0001004434-21-000041 | 10-Q | XBRL Viewer | Affiliated Managers Group INC

Affiliate Equity - Changes in the Company's Interest in its Affiliates on the Controlling Interest's Equity (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Affiliate Equity [Abstract]
Increase (decrease) in controlling interest paid-in capital from Affiliate equity issuances	$ 0.0	$ 2.9	$ (17.5)	$ 1.6
Decrease in controlling interest paid-in capital from Affiliate equity purchases	(1.2)	(65.4)	(57.2)	(226.0)
Net income (loss) (controlling interest) including the net impact of Affiliate equity transactions	$ 127.2	$ 8.8	$ 312.6	$ (138.1)

Label	Element	Value
Depreciation and Other Amortization	amg_DepreciationAndOtherAmortization	$ 4,600,000
Depreciation and Other Amortization	amg_DepreciationAndOtherAmortization	4,000,000.0
Depreciation and Other Amortization	amg_DepreciationAndOtherAmortization	14,700,000
Depreciation and Other Amortization	amg_DepreciationAndOtherAmortization	12,500,000
Noncontrolling Interest, Decrease from Transfers to Noncontrolling Interest Holders	amg_NoncontrollingInterestDecreasefromTransferstoNoncontrollingInterestHolders	200,000
Noncontrolling Interest, Decrease from Transfers to Noncontrolling Interest Holders	amg_NoncontrollingInterestDecreasefromTransferstoNoncontrollingInterestHolders	200,000
Noncontrolling Interest, Decrease from Transfers to Noncontrolling Interest Holders	amg_NoncontrollingInterestDecreasefromTransferstoNoncontrollingInterestHolders	100,000
Noncontrolling Interest, Decrease from Transfers to Noncontrolling Interest Holders	amg_NoncontrollingInterestDecreasefromTransferstoNoncontrollingInterestHolders	100,000
Noncontrolling Interest, Decrease from Transfers to Noncontrolling Interest Holders	amg_NoncontrollingInterestDecreasefromTransferstoNoncontrollingInterestHolders	5,600,000
Noncontrolling Interest, Decrease from Transfers to Noncontrolling Interest Holders	amg_NoncontrollingInterestDecreasefromTransferstoNoncontrollingInterestHolders	5,600,000
Noncontrolling Interest, Decrease from Transfers to Noncontrolling Interest Holders	amg_NoncontrollingInterestDecreasefromTransferstoNoncontrollingInterestHolders	3,900,000
Noncontrolling Interest, Decrease from Transfers to Noncontrolling Interest Holders	amg_NoncontrollingInterestDecreasefromTransferstoNoncontrollingInterestHolders	3,900,000
Deferred Tax Liabilities, Net, Decrease Resulting From Repurchase Of Debt	amg_DeferredTaxLiabilitiesNetDecreaseResultingFromRepurchaseOfDebt	6,200,000
Deferred Income Tax Expense Related to Intangibles	amg_DeferredIncomeTaxExpenseRelatedToIntangibles	27,000,000.0
Deferred Income Tax Expense Related to Intangibles	amg_DeferredIncomeTaxExpenseRelatedToIntangibles	12,000,000.0
Deferred Income Tax Expense Related to Intangibles	amg_DeferredIncomeTaxExpenseRelatedToIntangibles	(7,100,000)
Deferred Income Tax Expense Related to Intangibles	amg_DeferredIncomeTaxExpenseRelatedToIntangibles	51,900,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Purchases And Issuances	amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesAndIssuances	85,800,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Purchases And Issuances	amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesAndIssuances	3,300,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Purchases And Issuances	amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesAndIssuances	292,800,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Purchases And Issuances	amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchasesAndIssuances	86,300,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements And Reductions	amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlementsAndReductions	89,000,000.0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements And Reductions	amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlementsAndReductions	3,500,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements And Reductions	amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlementsAndReductions	238,600,000
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Settlements And Reductions	amg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlementsAndReductions	62,800,000
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	2,900,000
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	200,000
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	3,100,000
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	500,000
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	500,000
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	1,100,000
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	18,800,000
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	19,900,000
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	(16,700,000)
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	21,000,000.0
Noncontrolling Interest, Increase (Decrease) from Subsidiary Equity Issuance	amg_NoncontrollingInterestIncreaseDecreaseFromSubsidiaryEquityIssuance	4,300,000
Amortization and Impairments of Intangible Assets	amg_AmortizationandImpairmentsofIntangibleAssets	31,900,000
Amortization and Impairments of Intangible Assets	amg_AmortizationandImpairmentsofIntangibleAssets	8,900,000
Amortization and Impairments of Intangible Assets	amg_AmortizationandImpairmentsofIntangibleAssets	133,400,000
Amortization and Impairments of Intangible Assets	amg_AmortizationandImpairmentsofIntangibleAssets	25,300,000
Amortization and Impairments of Intangible Assets	amg_AmortizationandImpairmentsofIntangibleAssets	25,300,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	3,500,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	6,000,000.0
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	9,500,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	2,500,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	8,900,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	11,400,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	11,400,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	24,900,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	36,300,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	9,600,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	36,500,000
Affiliate Equity Compensation Expense, Including Adjustments and Portion Attributable to Noncontrolling Interest	amg_AffiliateEquityCompensationExpenseIncludingAdjustmentsandPortionAttributabletoNoncontrollingInterest	$ 46,100,000
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	(0.05)
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.08
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.03
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.01
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.08
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.05
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.14
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.16
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.15
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.15
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.17
Obligations to Related Parties, Measurement Input	amg_ObligationstoRelatedPartiesMeasurementInput	0.15
Due to Related Parties, Fair Value Disclosure	amg_DuetoRelatedPartiesFairValueDisclosure	$ 22,000,000.0
Due to Related Parties, Fair Value Disclosure	amg_DuetoRelatedPartiesFairValueDisclosure	0
Due to Related Parties, Fair Value Disclosure	amg_DuetoRelatedPartiesFairValueDisclosure	0
Due to Related Parties, Fair Value Disclosure	amg_DuetoRelatedPartiesFairValueDisclosure	22,000,000.0
Due to Related Parties, Fair Value Disclosure	amg_DuetoRelatedPartiesFairValueDisclosure	47,700,000
Due to Related Parties, Fair Value Disclosure	amg_DuetoRelatedPartiesFairValueDisclosure	0
Due to Related Parties, Fair Value Disclosure	amg_DuetoRelatedPartiesFairValueDisclosure	0
Due to Related Parties, Fair Value Disclosure	amg_DuetoRelatedPartiesFairValueDisclosure	47,700,000
Redeemable Noncontrolling Interest, Equity, Change In Redemption Value	amg_RedeemableNoncontrollingInterestEquityChangeInRedemptionValue	(50,700,000)
Redeemable Noncontrolling Interest, Equity, Change In Redemption Value	amg_RedeemableNoncontrollingInterestEquityChangeInRedemptionValue	(50,700,000)
Redeemable Noncontrolling Interest, Equity, Change In Redemption Value	amg_RedeemableNoncontrollingInterestEquityChangeInRedemptionValue	900,000
Redeemable Noncontrolling Interest, Equity, Change In Redemption Value	amg_RedeemableNoncontrollingInterestEquityChangeInRedemptionValue	900,000
Redeemable Noncontrolling Interest, Equity, Change In Redemption Value	amg_RedeemableNoncontrollingInterestEquityChangeInRedemptionValue	(18,600,000)
Redeemable Noncontrolling Interest, Equity, Change In Redemption Value	amg_RedeemableNoncontrollingInterestEquityChangeInRedemptionValue	(18,600,000)
Redeemable Noncontrolling Interest, Equity, Change In Redemption Value	amg_RedeemableNoncontrollingInterestEquityChangeInRedemptionValue	(176,900,000)
Redeemable Noncontrolling Interest, Equity, Change In Redemption Value	amg_RedeemableNoncontrollingInterestEquityChangeInRedemptionValue	(176,900,000)
Proceeds From Equity Method Investment, Return Of Capital	amg_ProceedsFromEquityMethodInvestmentReturnOfCapital	0
Proceeds From Equity Method Investment, Return Of Capital	amg_ProceedsFromEquityMethodInvestmentReturnOfCapital	3,400,000
Proceeds From Equity Method Investment, Return Of Capital	amg_ProceedsFromEquityMethodInvestmentReturnOfCapital	3,400,000
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	21,000,000.0
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	(2,200,000)
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	18,800,000
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	1,200,000
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	(2,400,000)
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	(1,200,000)
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	57,400,000
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	(13,500,000)
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	43,900,000
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	9,500,000
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	13,200,000
Noncontrolling Interest, Increase (Decrease) from Purchase of Interests	amg_NoncontrollingInterestIncreaseDecreasefromPurchaseofInterests	22,700,000
Marketable Securities, Fair Value Disclosure	amg_MarketableSecuritiesFairValueDisclosure	74,900,000
Marketable Securities, Fair Value Disclosure	amg_MarketableSecuritiesFairValueDisclosure	25,700,000
Marketable Securities, Fair Value Disclosure	amg_MarketableSecuritiesFairValueDisclosure	49,200,000
Marketable Securities, Fair Value Disclosure	amg_MarketableSecuritiesFairValueDisclosure	0
Marketable Securities, Fair Value Disclosure	amg_MarketableSecuritiesFairValueDisclosure	75,800,000
Marketable Securities, Fair Value Disclosure	amg_MarketableSecuritiesFairValueDisclosure	61,100,000
Marketable Securities, Fair Value Disclosure	amg_MarketableSecuritiesFairValueDisclosure	14,700,000
Marketable Securities, Fair Value Disclosure	amg_MarketableSecuritiesFairValueDisclosure	0
Investment Commitment, Amount	amg_InvestmentCommitmentAmount	114,100,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings and Other Comprehensive Income (Loss)	amg_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedinEarningsandOtherComprehensiveIncomeLoss	200,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings and Other Comprehensive Income (Loss)	amg_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedinEarningsandOtherComprehensiveIncomeLoss	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings and Other Comprehensive Income (Loss)	amg_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedinEarningsandOtherComprehensiveIncomeLoss	4,100,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings and Other Comprehensive Income (Loss)	amg_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedinEarningsandOtherComprehensiveIncomeLoss	(2,200,000)
Other Investments and Other Assets	amg_OtherInvestmentsandOtherAssets	230,900,000
Other Investments and Other Assets	amg_OtherInvestmentsandOtherAssets	218,100,000
Intangible Assets Net Excluding Goodwill Translation Adjustments	amg_IntangibleAssetsNetExcludingGoodwillTranslationAdjustments	$ (5,300,000)
Fair Value Assumptions, Average Projected Growth Rate Period	amg_FairValueAssumptionsAverageProjectedGrowthRatePeriod	5 years
Noncontrolling Interest, Increase (Decrease) from Contributions from Affiliate and Other	amg_NoncontrollingInterestIncreaseDecreaseFromContributionsFromAffiliateAndOther	$ (2,500,000)
Noncontrolling Interest, Increase (Decrease) from Contributions from Affiliate and Other	amg_NoncontrollingInterestIncreaseDecreaseFromContributionsFromAffiliateAndOther	(2,500,000)
Noncontrolling Interest, Increase (Decrease) from Contributions from Affiliate and Other	amg_NoncontrollingInterestIncreaseDecreaseFromContributionsFromAffiliateAndOther	4,900,000
Noncontrolling Interest, Increase (Decrease) from Contributions from Affiliate and Other	amg_NoncontrollingInterestIncreaseDecreaseFromContributionsFromAffiliateAndOther	4,900,000
Noncontrolling Interest, Increase (Decrease) from Contributions from Affiliate and Other	amg_NoncontrollingInterestIncreaseDecreaseFromContributionsFromAffiliateAndOther	3,600,000
Noncontrolling Interest, Increase (Decrease) from Contributions from Affiliate and Other	amg_NoncontrollingInterestIncreaseDecreaseFromContributionsFromAffiliateAndOther	3,600,000
Purchases of Securities by Affiliate Sponsored Consolidated Products	amg_PurchasesofSecuritiesbyAffiliateSponsoredConsolidatedProducts	83,500,000
Purchases of Securities by Affiliate Sponsored Consolidated Products	amg_PurchasesofSecuritiesbyAffiliateSponsoredConsolidatedProducts	91,100,000
Sale of Securities by Affiliate Sponsored Consolidated Products	amg_SaleofSecuritiesbyAffiliateSponsoredConsolidatedProducts	77,600,000
Sale of Securities by Affiliate Sponsored Consolidated Products	amg_SaleofSecuritiesbyAffiliateSponsoredConsolidatedProducts	$ 49,900,000
Share-Based Compensation Arrangement By Share-based Payment Award, Options, Period Increase (Decrease), Performance Condition Changes	amg_ShareBasedCompensationArrangementBySharebasedPaymentAwardOptionsPeriodIncreaseDecreasePerformanceConditionChanges	600,000
Fair Value, Investments, Entities That Calculate NAVs, Life of Fund	amg_FairValueInvestmentsEntitiesThatCalculateNAVsLifeofFund	15 years
Other Comprehensive Income (Loss), Net Investment And Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	amg_OtherComprehensiveIncomeLossNetInvestmentAndCashFlowHedgeGainLossReclassificationBeforeTax	$ 100,000
Other Comprehensive Income (Loss), Net Investment And Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	amg_OtherComprehensiveIncomeLossNetInvestmentAndCashFlowHedgeGainLossReclassificationBeforeTax	400,000
Other Comprehensive Income (Loss), Net Investment And Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	amg_OtherComprehensiveIncomeLossNetInvestmentAndCashFlowHedgeGainLossReclassificationBeforeTax	400,000
Other Comprehensive Income (Loss), Net Investment And Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	amg_OtherComprehensiveIncomeLossNetInvestmentAndCashFlowHedgeGainLossReclassificationBeforeTax	1,400,000
Other Comprehensive Income (Loss), Net Investment And Cash Flow Hedge, Gain (Loss), Before Reclassification And Tax	amg_OtherComprehensiveIncomeLossNetInvestmentAndCashFlowHedgeGainLossBeforeReclassificationAndTax	300,000
Other Comprehensive Income (Loss), Net Investment And Cash Flow Hedge, Gain (Loss), Before Reclassification And Tax	amg_OtherComprehensiveIncomeLossNetInvestmentAndCashFlowHedgeGainLossBeforeReclassificationAndTax	(1,900,000)
Other Comprehensive Income (Loss), Net Investment And Cash Flow Hedge, Gain (Loss), Before Reclassification And Tax	amg_OtherComprehensiveIncomeLossNetInvestmentAndCashFlowHedgeGainLossBeforeReclassificationAndTax	13,800,000
Other Comprehensive Income (Loss), Net Investment And Cash Flow Hedge, Gain (Loss), Before Reclassification And Tax	amg_OtherComprehensiveIncomeLossNetInvestmentAndCashFlowHedgeGainLossBeforeReclassificationAndTax	(900,000)
Equity Securities Without Readily Determinable Fair Value, Cost	amg_EquitySecuritiesWithoutReadilyDeterminableFairValueCost	8,500,000
Equity Securities Without Readily Determinable Fair Value, Cost	amg_EquitySecuritiesWithoutReadilyDeterminableFairValueCost	8,500,000
Finite Lived Intangible Asset, Accumulated Amortization, Foreign Currency Translation	amg_FiniteLivedIntangibleAssetAccumulatedAmortizationForeignCurrencyTranslation	(100,000)
Investment and Other Income (Loss)	amg_InvestmentAndOtherIncomeLoss	12,700,000
Investment and Other Income (Loss)	amg_InvestmentAndOtherIncomeLoss	37,500,000
Investment and Other Income (Loss)	amg_InvestmentAndOtherIncomeLoss	2,900,000
Investment and Other Income (Loss)	amg_InvestmentAndOtherIncomeLoss	$ 91,100,000
Share-based Compensation Arrangement By Share-based Payment Award, Equity Instruments Other Than Options, Period Increase (Decrease), Performance Condition Changes	amg_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsPeriodIncreaseDecreasePerformanceConditionChanges	(100,000)
Finite Lived Intangible Assets, Foreign Currency Translation Gain (Loss), Before Adjustment On Accumulated Amortization	amg_FiniteLivedIntangibleAssetsForeignCurrencyTranslationGainLossBeforeAdjustmentOnAccumulatedAmortization	$ (200,000)
Accumulated Other Comprehensive Income (Loss) Attributable to Parent, Net of Tax	amg_AccumulatedOtherComprehensiveIncomeLossAttributableToParentNetOfTax	(98,300,000)
Accumulated Other Comprehensive Income (Loss) Attributable to Parent, Net of Tax	amg_AccumulatedOtherComprehensiveIncomeLossAttributableToParentNetOfTax	(83,700,000)
Proceeds from (Payments for) Affiliate Equity Issuances and Repurchases	amg_ProceedsFromPaymentsForAffiliateEquityIssuancesAndRepurchases	(232,500,000)
Proceeds from (Payments for) Affiliate Equity Issuances and Repurchases	amg_ProceedsFromPaymentsForAffiliateEquityIssuancesAndRepurchases	(47,800,000)
Line of Credit Facility, Maximum Borrowing Capacity, Additional Amount	amg_LineofCreditFacilityMaximumBorrowingCapacityAdditionalAmount	500,000,000
Line of Credit Facility, Maximum Borrowing Capacity, Additional Amount	amg_LineofCreditFacilityMaximumBorrowingCapacityAdditionalAmount	75,000,000
Business Combination, Consideration Transferred, Deferred	amg_BusinessCombinationConsiderationTransferredDeferred	150,200,000
Marketable Securities, Unrealized Gains	amg_MarketableSecuritiesUnrealizedGains	5,500,000
Marketable Securities, Unrealized Gains	amg_MarketableSecuritiesUnrealizedGains	8,200,000
Marketable Securities, Cost	amg_MarketableSecuritiesCost	69,400,000
Marketable Securities, Cost	amg_MarketableSecuritiesCost	68,500,000
Income (Loss) from Continuing Operations before Income Taxes Attributable to Parent	amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent	103,800,000
Income (Loss) from Continuing Operations before Income Taxes Attributable to Parent	amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent	171,000,000.0
Income (Loss) from Continuing Operations before Income Taxes Attributable to Parent	amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent	120,000,000.0
Income (Loss) from Continuing Operations before Income Taxes Attributable to Parent	amg_IncomeLossFromContinuingOperationsBeforeIncomeTaxesAttributableToParent	540,500,000
Marketable Securities, Unrealized Losses	amg_MarketableSecuritiesUnrealizedLosses	0.0
Marketable Securities, Unrealized Losses	amg_MarketableSecuritiesUnrealizedLosses	900,000
Marketable Securities, Fair Value, Deconsolidation	amg_MarketableSecuritiesFairValueDeconsolidation	52,300,000
Marketable Securities, Fair Value, Deconsolidation	amg_MarketableSecuritiesFairValueDeconsolidation	19,000,000
Obligations And Payments To Acquire Interest In Subsidiaries And Affiliates	amg_ObligationsAndPaymentsToAcquireInterestInSubsidiariesAndAffiliates	$ 144,300,000
Term Of Conditional Right to Put Interest	amg_TermOfConditionalRighttoPutInterest	5 years
Term Of Conditional Right to Put Interest	amg_TermOfConditionalRighttoPutInterest	15 years
Income (Loss) from Equity Method Investments Before Amortization of Intangibles	amg_IncomeLossfromEquityMethodInvestmentsBeforeAmortizationofIntangibles	$ 218,900,000
Other Commitment Expected Payments	amg_OtherCommitmentExpectedPayments	15,000,000
Equity Method Investment, Intangible Asset Amortization and Impairments	amg_EquityMethodInvestmentIntangibleAssetAmortizationandImpairments	93,800,000
Other Commitment, Contingent Consideration, Option To Reduce Ownership Interest, Asset	amg_OtherCommitmentContingentConsiderationOptionToReduceOwnershipInterestAsset	25,000,000
Equity Method Investment, Realized Gain (Loss) on Foreign Currency Translation	amg_EquityMethodInvestmentRealizedGainLossonForeignCurrencyTranslation	25,800,000
Other Commitment, Contingent Consideration, Asset	amg_OtherCommitmentContingentConsiderationAsset	12,500,000
Noncontrolling Interest, Increase (decrease) Attributable to Consolidated Partnerships	amg_NoncontrollingInterestIncreasedecreaseAttributabletoConsolidatedPartnerships	(17,100,000)
Equity Method Investment, Other Increase (Decrease)	amg_EquityMethodInvestmentOtherIncreaseDecrease	(6,700,000)
Noncontrolling Interest, Decrease from Transfers to Other Liabilities	amg_NoncontrollingInterestDecreasefromTransferstoOtherLiabilities	86,300,000
Effective Income Tax Rate Reconciliation, Release Of Uncertain Tax Position	amg_EffectiveIncomeTaxRateReconciliationReleaseOfUncertainTaxPosition	(5,500,000)
Other Commitment, To Be Paid, After Year One	amg_OtherCommitmentToBePaidAfterYearOne	$ 147,500,000

Affiliate Equity - Summary of Affiliate Recognized and Unrecognized Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Affiliate Equity [Line Items]
Controlling interest	$ 2.8	$ 3.5	$ 9.9	$ 11.4
Non-controlling interests	8.9	6.0	36.5	24.9
Affiliate equity compensation expense	11.7	$ 9.5	46.4	$ 36.3
Unrecognized Affiliate Equity Expense [Abstract]
Controlling Interest	47.6		$ 47.6		$ 35.9
Remaining Life			4 years		5 years
Non-controlling Interests	$ 108.1		$ 108.1		$ 109.7

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Notes Details Affiliate Equity - Summary of Affiliate Recognized and Unrecognized Expense (Details) Affiliate Equity - Changes in the Company's Interest in its Affiliates on the Controlling Interest's Equity (Details) Uncategorized Uncategorized Items - amg-20210930_htm.xml All Reports