Insurance.

No definition available.

The annual costs accrued for reinsurance coverage during the period.

No definition available.

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

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

No definition available.

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

Special charges to increase asbestos and environmental reserves.

No definition available.

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

Stated amount that is the upper retention limit per entity policy.

No definition available.

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

Amount of risk reinsured in excess of stated retention per company policy.

No definition available.

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

Percentage of risk reinsured in excess of stated retention per entity policy.

No definition available.

Name:	us-gaap_ReinsuranceRetentionPolicyExcessRetentionPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_ProductOrServiceAxis=afg_CatastropheBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

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

Name:	us-gaap_ProductOrServiceAxis=afg_IncreasedInsuranceLossReservesRelatedToMedicalMalpracticeAndGeneralLiabilityLinesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Insurance - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended
Insurance - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Catastrophe bonds
Insurance [Line Items]
Catastrophe reinsurance coverage, excess retention percentage	95.00%
Catastrophe reinsurance coverage, excess retention amount	$ 200
Catastrophe reinsurance coverage, retention amount	$ 100
Catastrophe reinsurance coverage, annual cost					$ 11
Property and Casualty Insurance \| Other lines
Insurance [Line Items]
Special charges to increase asbestos and environmental reserves		$ 89	$ 36		$ 89	$ 36
Special charges related to the exit of certain lines of business within subsidiaries				$ 65
Property and Casualty Insurance \| Increased insurance loss reserves related to medical malpractice and general liability lines \| Other lines
Insurance [Line Items]
Special charges related to the exit of certain lines of business within subsidiaries						$ 57