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Commitment and Contingencies - Additional Information (Details) $ in Millions	Sep. 30, 2023 USD ($)
Commitments and Contingencies
Co-investment commitments in partnership	$ 151.7
Deferred payment obligation	65.0
Equity Method Investee
Commitments and Contingencies
Deferred payment obligation	57.3
Payments for Achievement of Financial Targets | Equity Method Investee
Commitments and Contingencies
Total contingent payments	198.1
Expected payments	13.0
Payment contingent on not meeting financial targets	12.5
Payment contingent on exercise of option to reduce ownership percentage	$ 25.0

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- Definition

-- None. No documentation exists for this element. --

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No definition available.

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Name:	amg_CommitmentAndContingenciesLineItems
Namespace Prefix:	amg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Definition

Deferred Payment Obligations, Liability

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No definition available.

+ Details

Name:	amg_DeferredPaymentObligationsLiability
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

The amount that the entity agreed to co-invest in certain investment partnerships where the entity serves as the general partner.

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No definition available.

+ Details

Name:	amg_InvestmentCommitmentAmount
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

Other Commitment, Contingent Consideration, Asset

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No definition available.

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Name:	amg_OtherCommitmentContingentConsiderationAsset
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Other Commitment, Contingent Consideration, Option To Reduce Ownership Interest, Asset

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No definition available.

+ Details

Name:	amg_OtherCommitmentContingentConsiderationOptionToReduceOwnershipInterestAsset
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Other Commitment Expected Payments

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No definition available.

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Name:	amg_OtherCommitmentExpectedPayments
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

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No definition available.

+ Details

Name:	us-gaap_OtherCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_EquityMethodInvesteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_OtherCommitmentsAxis=amg_PaymentsForAchievementOfFinancialTargetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: