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Commitments and Contingencies - Additional Information (Details) $ in Millions	Sep. 30, 2024 USD ($)
Commitments and Contingencies
Co-investment commitments in partnership	$ 215.7
Deferred payment obligation	43.3
Equity Method Investee
Commitments and Contingencies
Deferred payment obligation	5.1
Payments for Achievement of Financial Targets
Commitments and Contingencies
Payment contingent on not meeting financial targets	12.5
Payment contingent on exercise of option to reduce ownership percentage	25.0
Payments for Achievement of Financial Targets | Equity Method Investee
Commitments and Contingencies
Total contingent payments	243.1
Contingent payments in remainder of fiscal year	4.0
Contingent payments, 2025 through 2028	239.1
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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No definition available.

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Namespace Prefix:	amg_
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Balance Type:	na
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- Definition

Deferred Payment Obligations, Liability

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No definition available.

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Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
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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

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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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Balance Type:	debit
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- Definition

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

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No definition available.

+ Details

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

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- Definition

Other Commitment, To Be Paid, Year One Through Four

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No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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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

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

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- Definition

Amount of commitment classified as other to be paid in remainder of current fiscal year.

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No definition available.

+ Details

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- Details

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- Details

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- Details

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