Name:	afg_ShortdurationInsuranceContractsHistoricalClaimsDurationYearEightCumulative
Namespace Prefix:	afg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

Percentage on a cumulative basis of average payout, after reinsurance, in the fifth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

No definition available.

Name:	afg_ShortdurationInsuranceContractsHistoricalClaimsDurationYearFiveCumulative
Namespace Prefix:	afg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

Percentage on a cumulative basis of average payout, after reinsurance, in the fourth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

No definition available.

Name:	afg_ShortdurationInsuranceContractsHistoricalClaimsDurationYearFourCumulative
Namespace Prefix:	afg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

Percentage on a cumulative basis of average payout, after reinsurance, in the ninth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

No definition available.

Name:	afg_ShortdurationInsuranceContractsHistoricalClaimsDurationYearNineCumulative
Namespace Prefix:	afg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

Percentage on a cumulative basis of average payout, after reinsurance, in the seventh year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

No definition available.

Name:	afg_ShortdurationInsuranceContractsHistoricalClaimsDurationYearSevenCumulative
Namespace Prefix:	afg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

Percentage on a cumulative basis of average payout, after reinsurance, in the sixth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

No definition available.

Name:	afg_ShortdurationInsuranceContractsHistoricalClaimsDurationYearSixCumulative
Namespace Prefix:	afg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

Percentage on a cumulative basis of average payout, after reinsurance, in the tenth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

No definition available.

Name:	afg_ShortdurationInsuranceContractsHistoricalClaimsDurationYearTenCumulative
Namespace Prefix:	afg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

Percentage on a cumulative basis of average payout, after reinsurance, in the third year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

No definition available.

Name:	afg_ShortdurationInsuranceContractsHistoricalClaimsDurationYearThreeCumulative
Namespace Prefix:	afg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

Percentage on a cumulative basis of average payout, after reinsurance, in the second year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

No definition available.

Name:	afg_ShortdurationInsuranceContractsHistoricalClaimsDurationYearTwoCumulative
Namespace Prefix:	afg_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

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.

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

Percentage of average annual payout, after reinsurance, in the eighth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 9F
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-9F

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 4G
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-4G

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearEight
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of average annual payout, after reinsurance, in the fifth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearFive
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of average annual payout, after reinsurance, in the fourth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearFour
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of average annual payout, after reinsurance, in the ninth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearNine
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of average annual payout, after reinsurance, in the first year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearOne
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of average annual payout, after reinsurance, in the seventh year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearSeven
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of average annual payout, after reinsurance, in the sixth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearSix
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of average annual payout, after reinsurance, in the tenth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearTen
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of average annual payout, after reinsurance, in the third year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearThree
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Percentage of average annual payout, after reinsurance, in the second year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearTwo
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

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

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

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

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