Pretax COVID-19 related losses

No definition available.

Name:	afg_PretaxCOVID19RelatedLosses
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pretax Catastrophe losses

+ References

No definition available.

+ Details

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

- Definition

Quarterly operating results unaudited.

+ References

No definition available.

+ Details

Name:	afg_QuarterlyOperatingResultsUnauditedLineItems
Namespace Prefix:	afg_
Data Type:	xbrli:stringItemType
Balance Type:	na
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 of realized gain (loss) on investment in marketable security, including other-than-temporary impairment (OTTI).

+ References

No definition available.

+ Details

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

- Definition

Write-off of amounts previously capitalized as debt issuance cost in an extinguishment of debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_OtherAggregatedInvestmentsMember
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_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:

- Details

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

- Details

Name:	afg_QuarterlyOperatingResultsUnauditedAxis=afg_FormerRailroadAndManufacturingOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201601Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Quarterly Operating Results (Unaudited) - Narrative (Details) - USD ($) $ in Millions	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities										$ 289	$ 287	$ (266)
Pretax catastrophe losses	$ 38	$ 57	$ 26	$ 9	$ 15	$ 22	$ 12	$ 12		130	61
Pretax COVID-19 related losses			105	10						115
Other Security Investments
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities	384	15	2	(16)	(2)	(3)	12	2		385	9
Property and casualty insurance \| Other lines
Quarterly Operating Results (Unaudited) [Line Items]
Increase in asbestos and environmental reserves		47				18				47	18	18
Property and casualty insurance \| Other lines \| Neon Capital Limited
Quarterly Operating Results (Unaudited) [Line Items]
Pretax catastrophe losses										37	13
Pretax COVID-19 related losses									$ 20	20
Other
Quarterly Operating Results (Unaudited) [Line Items]
Write off of deferred debt issuance cost due to the retirement of debt	5				5
Other \| Former railroad and manufacturing operations
Quarterly Operating Results (Unaudited) [Line Items]
Increase in asbestos and environmental reserves		21				11
Annuity \| Other Security Investments
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities	369
Equity securities
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities										(96)	277	(265)
Equity securities \| Accounting Standards Update 2016-01
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities	$ 207	$ 30	$ 202	$ (535)	$ 67	$ (15)	$ 44	$ 182		(96)	278
Total fixed maturities
Quarterly Operating Results (Unaudited) [Line Items]
Realized gains (losses) on securities										$ 422	$ (3)	$ (20)