Asset-backed securities average useful life.

No definition available.

Name:	afg_AssetBackedSecuritiesAverageUsefulLife
Namespace Prefix:	afg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Mortgage backed securities average useful life.

+ References

No definition available.

+ Details

Name:	afg_MortgageBackedSecuritiesAverageUsefulLife
Namespace Prefix:	afg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Non-credit related portion of other-than-temporary impairment charges taken for securities still owned.

+ References

No definition available.

+ Details

Name:	afg_NonCreditRelatedPortionOfOttiChargesTakenForSecuritiesStillOwned
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of available-for-sale securities in an unrealized loss position.

+ References

No definition available.

+ Details

Name:	afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition
Namespace Prefix:	afg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage (based on fair value) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade.

+ References

No definition available.

+ Details

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

- Definition

Percentage based on unrealized loss of available for sale fixed maturities that are in an unrealized loss position and rated investment grade.

+ References

No definition available.

+ Details

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

- Definition

Line items represent reporting 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

+ Details

Name:	invest_InvestmentLineItems
Namespace Prefix:	invest_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

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

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

+ References

+ Details

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

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

+ Details

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

- 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_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPositionTwelveMonthsOrLongerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments - Narrative (Details) $ in Millions	12 Months Ended
Investments - Narrative (Details) $ in Millions	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Investment [Line Items]
Percentage (based on amount of unrealized loss) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade	84.00%
Percentage (based on fair value) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade	91.00%
Other than temporary impairment charges	$ 116	$ 125	$ 36
Average life of ABS	5 years
Average life of MBS	4 years 6 months
Residential MBS
Investment [Line Items]
Non-credit related portion of other-than-temporary impairment charges taken for securities still owned	$ 189	205
Fixed maturities, Gross Unrealized, Losses	(13)	(15)
Other than temporary impairment charges	3
Securities with non-credit other-than-temporary impairment charges
Investment [Line Items]
Gross Unrealized, Gains	130	134
Gross Unrealized, Losses	(3)	(6)
Fixed maturities
Investment [Line Items]
Fixed maturities, Gross Unrealized, Losses	$ (245)	(374)
Number of available for sale securities in an unrealized loss position \| security	1,242
Other than temporary impairment charges	$ 38	43	$ 15
Corporate bonds
Investment [Line Items]
Other than temporary impairment charges	23
Fixed maturities — all other
Investment [Line Items]
Other than temporary impairment charges	$ 12
Common stocks
Investment [Line Items]
Number of available for sale securities in an unrealized loss position \| security	23
Other than temporary impairment charges	$ 89
Equity securities, Gross Unrealized, Losses	$ (23)	(79)
Preferred stocks
Investment [Line Items]
Number of available for sale securities in an unrealized loss position \| security	24
Other than temporary impairment charges	$ 4
Equity securities, Gross Unrealized, Losses	$ (7)	$ (6)
Number of available for sale securities in an unrealized loss position for twelve months or longer \| Common stocks
Investment [Line Items]
Number of available for sale securities in an unrealized loss position \| security	0
Number of available for sale securities in an unrealized loss position for twelve months or longer \| Preferred stocks
Investment [Line Items]
Number of available for sale securities in an unrealized loss position \| security	1