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Commitments and Contingencies - Additional Information (Details) $ in Millions | Mar. 31, 2024 USD ($)
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Commitments and Contingencies |
Co-investment commitments in partnership | $ 195.3
Equity Method Investee |
Commitments and Contingencies |
Total contingent payments | 58.5
Payments for Achievement of Financial Targets |
Commitments and Contingencies |
Total contingent payments | 236.0
Contingent payments, payable in remainder of fiscal year | 4.0
Other commitment, payable in year two | 232.0
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

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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

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- 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

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- 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

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- Definition Other Commitment, Payable In Remainder Of Fiscal Year + References No definition available. + Details
Name: | amg_OtherCommitmentPayableInRemainderOfFiscalYear
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Other Commitment, Payable in Year Two Through Five + References No definition available. + Details
Name: | amg_OtherCommitmentPayableInYearTwoThroughFive
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

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- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_EquityMethodInvesteeMember
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: |

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- Details
Name: | us-gaap_OtherCommitmentsAxis=amg_PaymentsForContingentFinancingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |