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Equity Distribution Program (Details) - USD ($) | Sep. 30, 2022 | Jun. 30, 2022
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Equity Distribution Program |  |
Class of Stock [Line Items] |  |
Equity distribution program, maximum amount authorized | $ 0 | $ 500,000,000

X
- Definition Stock Issuable, Value, Maximum + References No definition available. + Details
Name: | amg_StockIssuableValueMaximum
Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ClassOfStockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_ForwardContractIndexedToIssuersEquityTypeAxis=amg_ForwardEquityAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |