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Accounting Policies (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Variable Interest Entity [Line Items] |  |  |
Assets | $ 3,279 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets |  | $ 3,108 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
Liabilities | 2,952 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities |  | 2,819 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
Weighted average common shares adjustment related to stock-based compensation | 1.8 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements | 2.0 us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements |
Anti-dilutive potential common shares related to stock-based compensation plans | 1.3 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 0.6 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount |
Maturities of short term investments | 3 months |  |
New collateralized loan obligation temporary warehousing entities [Member] |  |  |
Variable Interest Entity [Line Items] |  |  |
Assets | 222 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = afg_NewCollateralizedLoanObligationTemporaryWarehousingEntitiesMember |  |
Liabilities | $ 177 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = afg_NewCollateralizedLoanObligationTemporaryWarehousingEntitiesMember |  |

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