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Segments of Operations - Narrative (Details) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 segment | Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items] |  |
Number of segments | segment | 3 |
Property and Casualty Insurance | Real Estate Investment |  |
Segment Reporting Information [Line Items] |  |
Pretax income from the sale of real estate properties | $ |  | $ 13

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- Definition This element represents the amount of gain (loss) on sale of properties during the reporting period gross of the applicable income taxes realized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 974 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-15.(a)(1)(ii) -URI http://asc.fasb.org/extlink&oid=6875705&loc=d3e641549-123027 + Details
Name: | us-gaap_GainLossOnSaleOfPropertiesBeforeApplicableIncomeTaxes
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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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_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=afg_PropertyAndCasualtyInsuranceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_RealEstateInvestmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |