Segments of Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Total premiums earned	$ 946	$ 754
Net investment income	388us-gaap_NetInvestmentIncome	361us-gaap_NetInvestmentIncome
Other income	47us-gaap_OtherIncome	21us-gaap_OtherIncome
Revenues before realized gains (losses)	1,437afg_RevenuesBeforeRealizedGainsLosses	1,192afg_RevenuesBeforeRealizedGainsLosses
Realized gains on securities	19	19
Realized loss on subsidiaries	(162)afg_RealizedGainsLossesOnSubsidiaries	0afg_RealizedGainsLossesOnSubsidiaries
Total revenues	1,294us-gaap_Revenues	1,211us-gaap_Revenues
Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Net investment income	79	67
Other income	6	2
Revenues before realized gains (losses)	1,031	823
Annuity [Member]
Segment Reporting Information [Line Items]
Net investment income	292	275
Other income	24	18
Revenues before realized gains (losses)	316	293
Run-off Long-term Care and Life [Member]
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	46	51
Other [Member]
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	44	25
Specialty Property and transportation [Member]
Segment Reporting Information [Line Items]
Total premiums earned	313	301
Specialty casualty [Member]
Segment Reporting Information [Line Items]
Total premiums earned	490	313
Specialty financial [Member]
Segment Reporting Information [Line Items]
Total premiums earned	120	117
Specialty Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 23	$ 23

Realized gains (losses) on subsidiaries.

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- Definition

Aggregate revenue (including insurance premiums, investment income and other income) recognized during the period before realized gains (losses) on securities and subsidiaries.

+ References

No definition available.

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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 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 7

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Reflects the sum of all other revenue and income recognized by the entity in the period not otherwise specified in the income statement.

+ References

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of property and casualty premiums earned.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.1)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 45

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-Section 50

-Paragraph 1

-Subparagraph (b),(c)

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ 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 210.5-03.1)

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