Long-Term Debt - Schedule of long-term debt (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Summary of Carrying value of long-term debt
Principal	$ 1,945	$ 1,993
Discount and Issue Costs	(28)	(29)
Carrying Value	1,917	1,964
Senior Notes \| AFG
Summary of Carrying value of long-term debt
Principal	1,270	1,318
Discount and Issue Costs	(9)	(10)
Carrying Value	$ 1,261	1,308
Senior Notes \| 4.50% Senior Notes due June 2047 \| AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	4.50%
Summary of Carrying value of long-term debt
Principal	$ 590	590
Discount and Issue Costs	(2)	(2)
Carrying Value	$ 588	588
Senior Notes \| 3.50% Senior Notes due August 2026 \| AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	3.50%
Summary of Carrying value of long-term debt
Principal	$ 377	425
Discount and Issue Costs	(2)	(3)
Carrying Value	$ 375	422
Senior Notes \| 5.25% Senior Notes due April 2030 \| AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.25%
Summary of Carrying value of long-term debt
Principal	$ 300	300
Discount and Issue Costs	(5)	(5)
Carrying Value	295	295
Senior Notes \| Other \| AFG
Summary of Carrying value of long-term debt
Principal	3	3
Discount and Issue Costs	0	0
Carrying Value	3	3
Subordinated Debentures \| AFG
Summary of Carrying value of long-term debt
Principal	675	675
Discount and Issue Costs	(19)	(19)
Carrying Value	$ 656	656
Subordinated Debentures \| 4.50% Subordinated Debentures due September 2060 \| AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	4.50%
Summary of Carrying value of long-term debt
Principal	$ 200	200
Discount and Issue Costs	(5)	(5)
Carrying Value	$ 195	195
Subordinated Debentures \| 5.125% Subordinated Debentures due December 2059 \| AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.125%
Summary of Carrying value of long-term debt
Principal	$ 200	200
Discount and Issue Costs	(6)	(6)
Carrying Value	$ 194	194
Subordinated Debentures \| 5.625% Subordinated Debentures due June 2060 \| AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.625%
Summary of Carrying value of long-term debt
Principal	$ 150	150
Discount and Issue Costs	(4)	(4)
Carrying Value	$ 146	146
Subordinated Debentures \| 5.875% Subordinated Debentures due March 2059 \| AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.875%
Summary of Carrying value of long-term debt
Principal	$ 125	125
Discount and Issue Costs	(4)	(4)
Carrying Value	$ 121	$ 121

Summary of Carrying value of long-term debt.

No definition available.

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

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1D
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495340-112611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=124435984&loc=d3e28555-108399

+ Details

Name:	us-gaap_DeferredFinanceCostsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=123466204&loc=SL6031897-161870

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69C
-URI http://asc.fasb.org/extlink&oid=123466577&loc=SL123495737-112612

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69B
-URI http://asc.fasb.org/extlink&oid=123466577&loc=SL123495735-112612

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495340-112611

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=afg_SeniorNotesDueInJune2047Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=afg_SeniorNotesDueInAugust2026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=afg_SeniorNotesDueInApril2030Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=afg_OtherLongtermDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=afg_SubordinatedDebenturesDueInSeptember2060Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=afg_SubordinatedDebenturesdueinDecember2059Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=afg_SubordinatedDebenturesDueInJune2060Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=afg_SubordinatedDebenturesDueInMarch2059Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: