Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Commitments and Contingencies
Co-investment commitments in partnership	$ 285.0
Fair value of contingent payment obligations	0.0	$ 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	100.0
Equity Method Investee
Commitments and Contingencies
Deferred payment obligations	85.8
Deferred payment obligations, to be paid, year one	56.4
Deferred payment obligations, to be paid, year two	29.4
Payments for Achievement of Financial Targets
Commitments and Contingencies
Total contingent payments	451.7
Contingent payments, year one	4.0
Contingent payments, year two	366.4
Contingent payments, year three	35.9
Contingent payments, year four	22.7
Contingent payments, year five	$ 22.7

-- 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

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

Deferred Payment Obligations, Liability

+ References

No definition available.

+ Details

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

- Definition

Deferred Payment Obligations, To Be Paid, Year One

+ References

No definition available.

+ Details

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

- Definition

Deferred Payment Obligations, To Be Paid, Year Two

+ References

No definition available.

+ Details

Name:	amg_DeferredPaymentObligationsToBePaidYearTwo
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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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.

+ References

No definition available.

+ Details

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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 235
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477314/942-235-S99-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-4

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-3

+ 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 fifth 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

+ 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 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

+ Details

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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

+ Details

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- 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

+ Details

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- 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

+ 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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