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v3.20.1

Commitments and Contingencies (Details) - USD ($) | 3 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Mar. 31, 2020
Commitments and Contingencies |  |
Payment contingent on not meeting financial targets (up to) |  | $ 12,500,000
Payment contingent on excercise of option to reduce ownership percentage |  | 25,000,000.0
Affiliate |  |
Commitments and Contingencies |  |
Co-investment commitments in partnership |  | 127,000,000.0
Equity Method Investee |  |
Commitments and Contingencies |  |
Contingent payments through 2021 |  | 150,000,000.0
Contingent payments through 2022 |  | 77,500,000
Contingent payments through 2026 |  | 62,500,000
Expected payments |  | 48,000,000
Contingent payments in 2020 |  | $ 0
Noncontrolling interest, sell of ownership percentage | 5.00% |
Minority Owner | Equity Method Investee |  |
Commitments and Contingencies |  |
Ownership percentage by minority owner |  | 14.00%

X
- 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

X
- 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 Noncontrolling Interest, Notification of Possible Sale of Noncontrolling Interest, Percentage + References No definition available. + Details
Name: | amg_NoncontrollingInterestNotificationofPossibleSaleofNoncontrollingInterestPercentage
Namespace Prefix: | amg_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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, Due In Fourth, Fifth And Sixth Year + References No definition available. + Details
Name: | amg_OtherCommitmentDueInFourthFifthAndSixthYear
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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 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: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Minimum amount of other commitment maturing in the second fiscal year following the latest fiscal year for commitments not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in this 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_OtherCommitmentDueInSecondYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Minimum amount of other commitment maturing in the third fiscal year following the latest fiscal year for commitments not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in this 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_OtherCommitmentDueInThirdYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of other commitments maturing in the remainder of the fiscal year following the latest fiscal year ended. + References No definition available. + Details
Name: | us-gaap_OtherCommitmentsFutureMinimumPaymentsRemainderOfFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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

X
- Details
Name: | srt_OwnershipAxis=amg_MinorityOwnerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |