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v3.20.4

Additional Information - Narrative (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Allowance for reinsurance recoverables [Line Items] |  |  |
Commitments to fund limited partnerships | $ 993 |  |
Retirement and employee savings plan expense | $ 41 | $ 39 | $ 37
Great American Life Insurance Company |  |  |
Allowance for reinsurance recoverables [Line Items] |  |  |
Percent of commitments to fund limited partnerships related Great American Life Insurance Company | 0.50 |  |

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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_UnfundedCommitmentsToLimitedPartnerships
Namespace Prefix: | afg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Unfunded commitments to limited partnerships, percent related to subsidiary + References No definition available. + Details
Name: | afg_UnfundedCommitmentsToLimitedPartnershipsPercentRelatedToSubsidiary
Namespace Prefix: | afg_
Data Type: | xbrli:pureItemType
Balance Type: | na
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: | srt_ValuationAndQualifyingAccountsDisclosureLineItems
Namespace Prefix: | srt_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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

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- Details
Name: | srt_ConsolidatedEntitiesAxis=afg_GreatAmericanLifeInsuranceCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |