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Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions | Nov. 07, 2022 | Sep. 30, 2022
--- | --- | ---
Commitments and Contingencies |  |
Co-investment commitments in partnership |  | $ 154.7
Equity Method Investee |  |
Commitments and Contingencies |  |
Total contingent payments |  | 68.0
Equity Method Investee | Subsequent Event |  |
Commitments and Contingencies |  |
Total contingent payments | $ 187.7 |
Payments for Achievement of Financial Targets |  |
Commitments and Contingencies |  |
Total contingent payments |  | 147.5
Expected payments |  | 13.0
Payment contingent on not meeting financial targets (up to) |  | 12.5
Payment contingent on exercise of option to reduce ownership percentage |  | $ 25.0

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- 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

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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

X
- 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

X
- 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

X
- 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

X
- 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

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

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
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: |