Investments in individual issuers as percentage of shareholders equity.

No definition available.

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- Details

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- Definition

Mortgage backed securities average useful life.

+ References

No definition available.

+ Details

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Namespace Prefix:	afg_
Data Type:	xbrli:durationItemType
Balance Type:	na
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- 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
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Data Type:	xbrli:integerItemType
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- 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

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Data Type:	num:percentItemType
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- 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

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Namespace Prefix:	afg_
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- Definition

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

+ References

No definition available.

+ Details

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- 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

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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

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Investments (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Securities	Sep. 30, 2011	Sep. 30, 2012 Residential Mortgage Backed Securities [Member]	Dec. 31, 2011 Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on fixed maturities	$ (87)		$ (87)		$ (74)	$ (180)
Non-credit related portion of other-than-temporary impairment charges taken for securities still owned					228	227
Other than temporary impairment charges	8	10	19	41	5
Investments (Textual) [Abstract]
Number of available for sale fixed maturities in an unrealized loss position			420
Percentage (based on amount of unrealized loss) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade			22.00%
Percentage (based on fair value) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade			50.00%
Realized gain (loss) from the mark-to-market of certain MBS	$ 1	$ 3	$ 4	$ (1)
Average life of MBS			4 years
Percentage increase in shareholders' equity			10.00%