Business Combination, Consideration Transferred, Deferred

No definition available.

Name:	amg_BusinessCombinationConsiderationTransferredDeferred
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

- 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

Noncontrolling Interest, Notification of Possible Sale of Noncontrolling Interest, Percentage

+ References

No definition available.

+ Details

Name:	amg_NoncontrollingInterestNotificationofPossibleSaleofNoncontrollingInterestPercentage
Namespace Prefix:	amg_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Other Commitment, Contingent Consideration, Asset

+ References

No definition available.

+ Details

Name:	amg_OtherCommitmentContingentConsiderationAsset
Namespace Prefix:	amg_
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

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

- Definition

Other Commitment Expected Payments

+ References

No definition available.

+ Details

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

- Definition

Other Commitment, To Be Paid, After Year One

+ References

No definition available.

+ Details

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

- Definition

Amount of liability recognized arising from contingent consideration in a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 35
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116859824&loc=d3e6819-128478

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

+ Details

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

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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

Name:	us-gaap_OtherCommitment
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 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

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

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

+ Details

Name:	us-gaap_PaymentsToAcquireBusinessesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=amg_ParnassusMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_OwnershipAxis=amg_UndisclosedAffiliateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Commitments and Contingencies (Details) - USD ($)		3 Months Ended
Commitments and Contingencies (Details) - USD ($)	Oct. 01, 2021	Dec. 31, 2020	Sep. 30, 2021
Commitments and Contingencies
Co-investment commitments in partnership			$ 114,100,000
Parnassus \| Subsequent Event
Commitments and Contingencies
Payments to acquire businesses	$ 300,400,000
Consideration transferred, deferred	150,200,000
Contingent payment obligation (up to)	$ 75,100,000
Equity Method Investee
Commitments and Contingencies
Total contingent payments			103,900,000
Contingent payments in year one			$ 40,000,000
Noncontrolling interest, sell of ownership percentage		5.00%
Undisclosed Affiliate \| Equity Method Investee
Commitments and Contingencies
Ownership percentage by minority owner			14.00%
Payments for Achievement of Financial Targets
Commitments and Contingencies
Total contingent payments			$ 188,000,000
Contingent payments in year one			40,500,000
Contingent payments after year one			147,500,000
Expected payments			15,000,000
Payment contingent on not meeting financial targets (up to)			12,500,000
Payment contingent on exercise of option to reduce ownership percentage			$ 25,000,000