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Shareholders' Equity (Details Textual) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Net of tax unrealized gains (losses) related to credit-only impaired securities | $ 57 afg_NetOfTaxUnrealizedGainsLossesOnCreditOnlyImpairedSecurities |  | $ 58 afg_NetOfTaxUnrealizedGainsLossesOnCreditOnlyImpairedSecurities
Common Stock options issued under stock incentive plan, average exercise price | $ 63.15 us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice | $ 56.44 us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice |
Compensation expense related to stock incentive plans | $ 6 us-gaap_AllocatedShareBasedCompensationExpense | $ 7 us-gaap_AllocatedShareBasedCompensationExpense |
Stock Incentive Plan [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Restricted Common Stock, shares issued | 171,130 us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross / us-gaap_PlanNameAxis = us-gaap_StockCompensationPlanMember |  |
Restricted Common Stock, fair value per share | $ 63.15 afg_RestrictedCommonStockIssuedFairValuePerShare / us-gaap_PlanNameAxis = us-gaap_StockCompensationPlanMember |  |
Common Stock options issued under stock incentive plan | 716,818 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod / us-gaap_PlanNameAxis = us-gaap_StockCompensationPlanMember |  |
Common Stock options issued under stock incentive plan, average exercise price | $ 63.15 us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice / us-gaap_PlanNameAxis = us-gaap_StockCompensationPlanMember |  |
Annual Equity Bonus Plan [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Other benefit plans, shares | 54,732 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation / us-gaap_PlanNameAxis = afg_AnnualEquityBonusPlanMember |  |
Common Stock issued, fair value per share | $ 62.55 afg_CommonStockIssuedDuringPeriodFairValuePerShare / us-gaap_PlanNameAxis = afg_AnnualEquityBonusPlanMember |  |

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- Definition The net of tax unrealized gains or losses related to securities for which only the credit portion of an other-than-temporary impairment has been recorded in earnings. + References No definition available. + Details
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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition Net number of share options (or share units) granted during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Total number of shares issued during the period, including shares forfeited, as a result of Restricted Stock Awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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