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Fair Value Measurements - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) | Dec. 31, 2020 USD ($) professional | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Fair Value Measurements (Textual) [Abstract] |  |  |  |
AFG's internal investment professionals | professional |  | 20 |  |
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 |  | 40.00% |  |
Level 3 assets that were priced using non-binding broker quotes |  | $ 1,500 |  |
Level three assets that were priced by pricing services. |  | $ 530 |  |
Percentage Of level three assets that were priced by pricing services |  | 14.00% |  |
Percentage of internally developed level three assets compared to total level 3 assets |  | 46.00% |  |
Fair value of equity securities transferred into level 3 |  | $ 620 | $ 244 | $ 270
Life contingent annuities |  | 1,110 | 247 |
Equity securities |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Fair value of equity securities transferred into level 3 | $ 29 | 17 | 56 | 30
Fixed-indexed and variable-indexed annuities (embedded derivative) |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Fair value of derivatives in annuity benefits accumulated measured using a discounted cash flow approach |  | $ 3,930 |  |
Fixed-indexed and variable-indexed annuities (embedded derivative) | Minimum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  | 4.00% |  |
Fixed-indexed and variable-indexed annuities (embedded derivative) | Maximum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  | 23.00% |  |
Fixed-indexed annuities (embedded derivative), majority of future years | Minimum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  | 8.00% |  |
Fixed-indexed annuities (embedded derivative), majority of future years | Maximum |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Unobservable input surrenders used in Level 3 fair value determination |  | 11.00% |  |
Not Designated as Hedging Instrument | Annuity benefits | Fixed-indexed and variable-indexed annuities (embedded derivative) |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Total realized/unrealized gains (losses) included in net income for the embedded derivatives related to the unlocking of actuarial assumptions |  | $ 240 | $ 181 | $ (44)
Level 3 |  |  |  |
Fair Value Measurements (Textual) [Abstract] |  |  |  |
Internally developed level 3 assets |  | $ 1,680 |  |

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