Property and casualty insurance revenues derived from customers of foreign countries.

No definition available.

Name:	afg_DisclosureOnGeographicAreasPropertyAndCasualtyRevenuesAttributedToForeignCountries
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Favorable prior year reserve development of the Company's liability for losses and loss adjustment expenses related to certain lines of business within subsidiaries including exited and ongoing lines of business in connection with a reinsurance to close agreement.

+ References

No definition available.

+ Details

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

- Definition

Special charges related to the exit of certain lines of business within subsidiaries.

+ References

No definition available.

+ Details

Name:	afg_SpecialChargesRelatedToTheExitOfCertainLinesOfBusinessWithinSubsidiaries
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

This element represents the amount of gain (loss) on sale of properties during the reporting period gross of the applicable income taxes realized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 974
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-15.(a)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=6875705&loc=d3e641549-123027

+ Details

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

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=114001942&loc=d3e12355-112629

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114001942&loc=d3e12317-112629

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainsLossesOnExtinguishmentOfDebt
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

No definition available.

+ 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_StatementBusinessSegmentsAxis=afg_PropertyAndCasualtyInsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_RealEstateInvestmentMember
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:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember
Namespace Prefix:
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Period Type:

Segments of Operations - Narrative (Details) $ in Millions	3 Months Ended				12 Months Ended
Segments of Operations - Narrative (Details) $ in Millions	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of segments \| segment					3
Revenue derived from sales of property and casualty insurance outside of the United States					7.00%	5.00%	4.00%
Property and casualty insurance \| Real estate investment
Segment Reporting Information [Line Items]
Pretax income from the sale of real estate properties included in other income				$ 13			$ 32
Property and casualty insurance \| Other lines
Segment Reporting Information [Line Items]
Special charges to increase asbestos and environmental reserves	$ 18		$ 89		$ 18	$ 89	36
Favorable reserve development related to certain lines of business within subsidiaries in connection with a reinsurance to close agreement						18
Special charges related to the exit of certain lines of business within subsidiaries							65
Other
Segment Reporting Information [Line Items]
Special charges to increase asbestos and environmental reserves					$ 9	24	$ 5
Make-whole premium, settlement of interest rate swap and write off of deferred debt issuance cost included in the loss on the retirement of debt		$ 40				$ 51