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Commitments and Contingencies - Additional Information (Details) $ in Millions	Mar. 31, 2024 USD ($)
Commitments and Contingencies
Co-investment commitments in partnership	$ 195.3
Equity Method Investee
Commitments and Contingencies
Total contingent payments	58.5
Payments for Achievement of Financial Targets
Commitments and Contingencies
Total contingent payments	236.0
Contingent payments, payable in remainder of fiscal year	4.0
Other commitment, payable in year two	232.0
Payment contingent on not meeting financial targets (up to)	12.5
Payment contingent on exercise of option to reduce ownership percentage	25.0
Payments for Contingent Financing
Commitments and Contingencies
Total contingent payments	$ 50.0

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- Definition

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

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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

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.

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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

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- Definition

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

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No definition available.

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

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- Definition

Other Commitment, Payable In Remainder Of Fiscal Year

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No definition available.

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

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- Definition

Other Commitment, Payable in Year Two Through Five

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No definition available.

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Name:	amg_OtherCommitmentPayableInYearTwoThroughFive
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.

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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:

X

- Details

Name:	us-gaap_OtherCommitmentsAxis=amg_PaymentsForContingentFinancingMember
Namespace Prefix:
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Balance Type:
Period Type: