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Investments (Details Textual) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 investment | Mar. 31, 2014 | Dec. 31, 2014 investment
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 | 55.00%afg_PercentageBasedOnUnrealizedLossOfAvailableForSaleFixedMaturitiesThatAreInUnrealizedLossPositionAndRatedInvestmentGrade |  |
Percentage (based on fair value) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade | 79.00%afg_PercentageBasedOnFairValueOfAvailableForSaleFixedMaturitiesThatAreInUnrealizedLossPositionAndRatedInvestmentGrade |  |
Other than temporary impairment charges | $ 4 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities | $ 1 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities |
Average life of ABS | 4 years 6 months |  |
Average life of MBS | 4 years 6 months |  |
The number of investments in individual issuers that exceeded 10% of Shareholders’ Equity | 0 afg_NumberOfInvestmentsInIndividualIssuersThatExceededOneTenthOfShareholdersEquity |  | 0 afg_NumberOfInvestmentsInIndividualIssuersThatExceededOneTenthOfShareholdersEquity
Percentage increase in shareholders' equity | 10.00%afg_InvestmentsInIndividualIssuersAsPercentageOfShareholdersEquity |  | 10.00%afg_InvestmentsInIndividualIssuersAsPercentageOfShareholdersEquity
Residential Mortgage Backed Securities [Member] |  |  |
Investment [Line Items] |  |  |
Non-credit related portion of other-than-temporary impairment charges taken for securities still owned | 218 afg_NonCreditRelatedPortionOfOttiChargesTakenForSecuritiesStillOwned / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  | 220 afg_NonCreditRelatedPortionOfOttiChargesTakenForSecuritiesStillOwned / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Fixed maturities, Gross Unrealized, Losses | (13)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |  | (15)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Securities with non-credit other-than-temporary impairment charges [Member] |  |  |
Investment [Line Items] |  |  |
Gross Unrealized, Gains | 152 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = afg_SecuritiesWithNonCreditOtherThanTemporaryImpairmentChargesMember |  | 151 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = afg_SecuritiesWithNonCreditOtherThanTemporaryImpairmentChargesMember
Gross Unrealized, Losses | (7)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = afg_SecuritiesWithNonCreditOtherThanTemporaryImpairmentChargesMember |  | (8)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = afg_SecuritiesWithNonCreditOtherThanTemporaryImpairmentChargesMember
Fixed maturities [Member] |  |  |
Investment [Line Items] |  |  |
Fixed maturities, Gross Unrealized, Losses | (54)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember |  | (79)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Number of available for sale securities in an unrealized loss position | 480 afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember |  |
Other than temporary impairment charges | 5 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 1 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember |
Corporate bonds [Member] |  |  |
Investment [Line Items] |  |  |
Other than temporary impairment charges | 5 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember |  |
Common stocks [Member] |  |  |
Investment [Line Items] |  |  |
Number of available for sale securities in an unrealized loss position | 33 afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |  |
Other than temporary impairment charges | 2 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |  |
Equity securities, Gross Unrealized, Losses | (33)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |  | (25)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Preferred stocks [Member] |  |  |
Investment [Line Items] |  |  |
Number of available for sale securities in an unrealized loss position | 11 afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |
Equity securities, Gross Unrealized, Losses | (3)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  | (5)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember
Maximum [Member] | Residential Mortgage Backed Securities [Member] |  |  |
Investment [Line Items] |  |  |
Other than temporary impairment charges | $ 1 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |
Number of available for sale securities in an unrealized loss position for twelve months or longer [Member] | Common stocks [Member] |  |  |
Investment [Line Items] |  |  |
Number of available for sale securities in an unrealized loss position | 0 afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition / us-gaap_InvestmentTypeAxis = afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPositionTwelveMonthsOrLongerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |  |
Number of available for sale securities in an unrealized loss position for twelve months or longer [Member] | Preferred stocks [Member] |  |  |
Investment [Line Items] |  |  |
Number of available for sale securities in an unrealized loss position | 7 afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition / us-gaap_InvestmentTypeAxis = afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPositionTwelveMonthsOrLongerMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |

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- Definition Investments in individual issuers as percentage of shareholders equity. + References No definition available. + Details
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