Asset-backed securities average useful llfe.

No definition available.

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

- Definition

Investments in individual issuers as percentage of shareholders equity.

+ References

No definition available.

+ Details

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

- Details

Name:	afg_InvestmentsTextualAbstract
Namespace Prefix:	afg_
Data Type:	xbrli:stringItemType
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

Realized gain (loss) from mark to market of certain mortgage backed securities.

+ References

No definition available.

+ Details

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

- Definition

This item represents the gross unrealized losses for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLosses1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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 FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 7

-Footnote 2

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15D

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15C

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Investments (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Securities	Mar. 31, 2012	Mar. 31, 2013 Residential Mortgage Backed Securities [Member]	Dec. 31, 2012 Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on fixed maturities	$ (52)		$ (38)	$ (54)
Non-credit related portion of other-than-temporary impairment charges taken for securities still owned			227	227
Other than temporary impairment charges		4
Investments (Textual) [Abstract]
Number of available for sale fixed maturities in an unrealized loss position	380
Percentage (based on amount of unrealized loss) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade	33.00%
Percentage (based on fair value) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade	73.00%
Average life of ABS	5 years
Average life of MBS	4 years
Percentage increase in shareholders' equity	10.00%
Realized gain (loss) from the mark-to-market of certain MBS	$ 2	$ 4