Earnings from real-estate related investments, net of DAC

No definition available.

Name:	afg_EarningsFromRealEstateRelatedInvestmentsNetOfDAC
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Net investment income plus other income less other expenses.

+ References

No definition available.

+ Details

Name:	afg_InvestmentAndOtherIncomeNet
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Underwriting income (loss) for the property and casualty insurance segment, which is calculated as premiums earned less losses and loss adjustment expenses less commissions and other underwriting expenses.

+ References

No definition available.

+ Details

Name:	afg_PropertyAndCasualtyInsuranceUnderwritingIncomeLoss
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Special charges to increase asbestos and environmental reserves

+ References

No definition available.

+ Details

Name:	afg_SpecialChargesToIncreaseAsbestosAndEnvironmentalReserves
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, after effects of reinsurance, of expense for claims incurred and costs incurred in the claim settlement process attributable to asbestos and environmental claims.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

+ Details

Name:	us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

+ Details

Name:	us-gaap_NetInvestmentIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e8672-108599

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=afg_NeonCapitalLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementOperatingActivitiesSegmentAxis=us-gaap_SegmentContinuingOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_RealEstateFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsegmentsAxis=afg_OtherLinesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=afg_NeonCapitalLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Segments of Operations - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended
Segments of Operations - Narrative (Details) $ in Millions	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of segments \| segment			2
Net investment income	$ 169	$ 122	$ 521	$ 314
Neon Capital Limited
Segment Reporting Information [Line Items]
Net investment income		1		(5)
Continuing operations \| Real estate-related investments
Segment Reporting Information [Line Items]
Net investment income		10	51	29
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net investment income	165	112	467	277
Property and casualty insurance underwriting	168	18	454	114
Investment and other income, net	161	100	451	249
Other
Segment Reporting Information [Line Items]
Special charges to increase asbestos and environmental reserves		21
Other \| Continuing operations \| Real estate-related investments
Segment Reporting Information [Line Items]
Net investment income	0	10	51	29
Earnings from real-estate related investments, net of DAC	0	3	51	11
Other lines \| Property and Casualty Insurance
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting	$ (1)	(86)	(2)	(133)
Special A&E charges			$ 0	47
Other lines \| Property and Casualty Insurance \| Neon Capital Limited
Segment Reporting Information [Line Items]
Property and casualty insurance underwriting		(38)		(82)
Investment and other income, net		(2)		(10)
Neon Capital Limited \| Other lines \| Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net investment income		$ 1		$ (5)