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Additional Information - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |  |
Commitments to fund credit facilities and limited partnerships | $ 578 |  |
Retirement and employee savings plan expense | $ 68 | $ 63 | $ 41

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- Definition Represents commitments to fund credit facilities and contribute capital to certain limited partnership investments at a future date. + References No definition available. + Details
Name: | afg_UnfundedCommitmentsToCreditFacilitiesAndLimitedPartnerships
Namespace Prefix: | afg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of cost (reversal of cost) for other postretirement benefits. Excludes pension benefits. + References No definition available. + Details
Name: | us-gaap_OtherPostretirementBenefitExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration