Commitments and Contingencies - Additional Information (Details) $ in Millions	Dec. 31, 2024 USD ($)
Commitments and Contingencies
Co-investment commitments in partnership	$ 236.5
Deferred payment obligations	4.7
Fair value of contingent payment obligations	5.7
Maximum \| Co-Investor
Commitments and Contingencies
Fair value of contingent payment obligations	24.9
Controlling Interest \| Maximum
Commitments and Contingencies
Fair value of contingent payment obligations	110.0
Equity Method Investee
Commitments and Contingencies
Deferred payment obligations	4.0
Payments for Achievement of Financial Targets
Commitments and Contingencies
Total contingent payments	239.4
Contingent payments, year one	118.8
Contingent payments, year two	23.0
Contingent payments, year three	75.8
Contingent payments, year four	21.8
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

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

No definition available.

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

- Definition

Contingent Consideration, Liability

+ References

No definition available.

+ Details

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

- Definition

Deferred Payment Obligations, Liability

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Other Commitment, Contingent Consideration, Asset

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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

- 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.

+ References

No definition available.

+ Details

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

- Definition

Amount of commitment classified as other to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of commitment classified as other to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of commitment classified as other to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of commitment classified as other to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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