Managed Investment Entities - Selected financial information related to CLOs (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Gains (losses) on change in fair value of assets/liabilities:
Assets	$ 10	$ 132	$ 77	$ (184)
Liabilities	(9)	(137)	(68)	163
Management fees paid to AFG	4	3	12	11
CLO earnings (losses) attributable to AFG shareholders	5	13	37	(21)
Variable interest entity, primary beneficiary
Variable Interest Entity [Line Items]
Investment in CLO tranches	96	176	96	176	$ 200
Variable interest entity, primary beneficiary \| Continuing operations
Variable Interest Entity [Line Items]
Investment in CLO tranches	96	47	96	47
Gains (losses) on change in fair value of assets/liabilities:
CLO earnings (losses) attributable to AFG shareholders	5	4	17	(7)
Variable interest entity, primary beneficiary \| Net earnings (loss) from discontinued operations \| Annuity subsidiaries
Variable Interest Entity [Line Items]
Investment in CLO tranches	0	129	0	129	$ 143
Gains (losses) on change in fair value of assets/liabilities:
CLO earnings (losses) attributable to AFG shareholders	$ 0	$ 9	$ 20	$ (14)

Earnings (losses) of variable interest entities attributable to AFG shareholders.

No definition available.

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

- Definition

Gain (loss) on change in fair value of assets of variable interest entities.

+ References

No definition available.

+ Details

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

- Definition

Gain (loss) on change in fair value of liabilities of variable interest entities.

+ References

No definition available.

+ Details

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

- Definition

Gains losses on change in fair value of assets liabilities collateralized loan obligations.

+ References

No definition available.

+ Details

Name:	afg_GainsLossesOnChangeInFairValueOfAssetsLiabilitiesOfCollateralizedLoanObligationsAbstract
Namespace Prefix:	afg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Management fees paid by variable interest entities.

+ References

No definition available.

+ Details

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

- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123419778&loc=d3e5747-111685

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=123419778&loc=d3e5728-111685

+ Details

Name:	us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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

- Details

Name:	srt_ConsolidatedEntitiesAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
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_StatementOperatingActivitiesSegmentAxis=us-gaap_SegmentDiscontinuedOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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