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Fair Value Measurements - Narrative (Details) | 3 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2023 USD ($) professional | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill impairment loss | $ 26,000,000 |  |
AFG's internal investment professionals | professional | 20 |  |
Percentage of level three assets that were priced using a discounted cash flow approach | 68.00% |  |
Level three assets that were priced using a discounted cash flow approach | $ 481,000,000 |  |
Percentage of level three assets, equity securities, priced using broker quotes and internal models | 18.00% |  |
Level three assets, equity securities, priced using broker quotes and internal models | $ 124,000,000 |  |
Verikai, Inc. |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Contingent consideration possible to be paid, maximum |  |  | $ 50,000,000
Contingent consideration — acquisitions |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of potential contingent consideration | 2,000,000 | $ 25,000,000 |
Contingent consideration — acquisitions | Verikai, Inc. |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of potential contingent consideration | 0 | 22,000,000 |
Change in fair value of contingent consideration liability | $ 22,000,000 |  |
Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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 | 10.00% |  |
Level 3 assets that were priced using non-binding broker quotes | $ 113,000,000 |  |
Percentage of level 3 assets that were priced by pricing services | 1.00% |  |
Level 3 assets that were priced by pricing services. | $ 13,000,000 |  |
Percentage of level 3 equity investments that do not qualify for equity accounting | 28.00% |  |
Level 3 assets that are equity securities that do not qualify for equity accounting | $ 322,000,000 |  |
Percentage of internally developed level 3 assets compared to total level 3 assets | 61.00% |  |
Internally developed level 3 assets | $ 706,000,000 |  |
Level 3 | Contingent consideration — acquisitions |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of potential contingent consideration | $ 2,000,000 | $ 25,000,000 |

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