Investments (Details 6) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments
Realized before impairments	$ 23	$ 29	$ 57	$ 52
Realized — impairments	(39)	(30)	(91)	(34)
Total realized gains (losses) on securities	(16)	(1)	(34)	18
Fixed maturities [Member]
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments
Realized before impairments	17	7	31	10
Realized — impairments	(19)	(10)	(35)	(15)
Total realized gains (losses) on securities	(2)	(3)	(4)	(5)
Change in unrealized	584	(623)	1,037	(405)
Equity securities [Member]
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments
Realized before impairments	9	25	32	46
Realized — impairments	(26)	(23)	(67)	(25)
Total realized gains (losses) on securities	(17)	2	(35)	21
Change in unrealized	11	(21)	(12)	(15)
Mortgage loans and other investments [Member]
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments
Realized before impairments	0	(2)	0	(2)
Realized — impairments	0	0	0	0
Total realized gains (losses) on securities	0	(2)	0	(2)
Change in unrealized	0	0	0	0
Other, Including DPAC and reserves on annuities and long-term care [Member]
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments
Realized before impairments	(3)	(1)	(6)	(2)
Realized — impairments	6	3	11	6
Total realized gains (losses) on securities	3	2	5	4
Change in unrealized	(253)	314	(473)	177
Total pretax [Member]
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments
Realized before impairments	23	29	57	52
Realized — impairments	(39)	(30)	(91)	(34)
Total realized gains (losses) on securities	(16)	(1)	(34)	18
Change in unrealized	342	(330)	552	(243)
Tax effects [Member]
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments
Realized before impairments	(8)	(10)	(20)	(18)
Realized — impairments	14	11	33	12
Total realized gains (losses) on securities	6	1	13	(6)
Change in unrealized	(119)	115	(193)	85
Noncontrolling interests [Member]
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments
Realized before impairments	(1)	0	(1)	0
Realized — impairments	1	0	2	0
Total realized gains (losses) on securities	0	0	1	0
Change in unrealized	(4)	3	(6)	2
Marketable securities [Member]
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments
Realized before impairments	14	19	36	34
Realized — impairments	(24)	(19)	(56)	(22)
Total realized gains (losses) on securities	(10)	0	(20)	12
Change in unrealized	$ 219	$ (212)	$ 353	$ (156)

Change in unrealized appreciation depreciation.

No definition available.

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

- Definition

Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments.

+ References

No definition available.

+ Details

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

- Definition

This item represents the total realized gain (loss) included in earnings for the period as a result of selling marketable securities categorized as trading, available-for-sale, or held-to-maturity. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27405-111563

+ Details

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

- Definition

This item represents the total realized gain (loss) included in earnings for the period as a result of selling marketable securities categorized as trading, available-for-sale, or held-to-maturity.

+ References

No definition available.

+ Details

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

- Definition

The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

+ Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=afg_MortgageLoansAndOtherInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=afg_OtherIncludingDPACandReservesonAnnuityandLongtermCareMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=afg_PretaxMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=afg_TaxEffectsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_NoncontrollingInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=afg_MarketableSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: