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Fair Value Measurements - Narrative (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 USD ($) stock | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Sep. 30, 2017 USD ($) stock | Sep. 30, 2018 USD ($) stock professional | Sep. 30, 2017 USD ($) stock | Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
AFG's internal investment professionals | professional |  |  |  |  | 25 |  |
Number of stocks transferred from Level 1 to Level 2 | stock | 0 |  |  |  |  |  |
Number of stocks transferred from Level 2 to Level 1 | stock | 0 |  |  |  |  |  |
Level 3 assets as a percentage of total assets measured at fair value |  |  |  |  | 7.00% |  |
Percentage of level 3 assets that were priced using non-binding broker quotes |  |  |  |  | 68.00% |  |
Level 3 assets that were priced using non-binding broker quotes |  |  |  |  | $ 2,180 |  |
Fair value of equity securities transferred into Level 3 | $ 13 |  |  | $ 188 | 100 | $ 277 |
Life contingent annuities | 229 |  |  |  | $ 229 |  | $ 206
Maximum |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Percentage of internally developed Level 3 asset fair values to Shareholders' equity (are less than) |  |  |  |  | 18.00% |  |
Fixed-indexed and variable-indexed annuities (embedded derivative), majority of future years | Minimum |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  |  |  |  | 7.00% |  |
Fixed-indexed and variable-indexed annuities (embedded derivative), majority of future years | Maximum |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  |  |  |  | 11.00% |  |
Fixed-indexed and variable-indexed annuities (embedded derivative) |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Fair value of derivatives in annuity benefits accumulated measured using a discounted cash flow approach | 3,110 |  |  |  | $ 3,110 |  |
Fixed-indexed and variable-indexed annuities (embedded derivative) | Minimum |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  |  |  |  | 3.00% |  |
Fixed-indexed and variable-indexed annuities (embedded derivative) | Maximum |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  |  |  |  | 23.00% |  |
Equity securities |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Fair value of equity securities transferred into Level 3 | 0 |  | $ 29 | $ 0 | $ 30 | $ 0 |
Preferred stocks |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Number of stocks transferred from Level 1 to Level 2 | stock |  |  |  |  | 2 |  |
Fair Value of assets transferred from Level 1 to Level 2, during the first nine months | $ 6 |  |  |  | $ 6 |  |
Number of stocks transferred from Level 2 to Level 1 | stock |  |  |  | 1 |  | 3 |
Fair Value of assets transferred from Level 2 to Level 1, during the third quarter |  |  |  | $ 1 |  |  |
Fair Value of assets transferred from Level 2 to Level 1, during the first nine months |  |  |  | $ 17 |  | $ 17 |
Not Designated as Hedging Instrument | Annuity benefits | Fixed-indexed annuities (embedded derivative) |  |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |
Total realized/unrealized gains (losses) included in net income for the embedded derivatives related to the unlocking of actuarial assumptions |  | $ (44) |  |  | $ (44) |  |

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