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Fair Value Measurements - Narrative (Details)
 3 Months Ended 12 Months Ended
Sep. 30, 2023
USD ($)
Dec. 31, 2025
USD ($)
professional
Dec. 31, 2023
USD ($)
Dec. 31, 2024
USD ($)
Dec. 31, 2021
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Goodwill, impairment loss $ 26,000,000   $ 26,000,000    
AFG's internal investment professionals | professional   25      
Level 3 assets as a percentage of total assets measured at fair value   6.00%      
Percentage of internally developed Level 3 assets compared to total Level 3 assets   88.00%      
Internally developed Level 3 assets   $ 883,000,000      
Percentage of Level 3 assets that were priced using a discounted cash flow approach   70.00%      
Level 3 assets that were priced using a discounted cash flow approach   $ 614,000,000      
Level 3 assets that were priced using unobservable internal models and third party inputs   $ 269,000,000      
Percentage of level 3 assets that were priced using financial information from third parties   9.00%      
Level 3 assets that were priced using financial information from third parties   $ 91,000,000      
Percentage of level 3 assets that were priced using non-binding broker quotes or pricing services   3.00%      
Level 3 assets that were priced using non-binding broker quotes or pricing services   $ 34,000,000      
Contingent consideration — acquisitions          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Contingent consideration — acquisitions   $ 3,000,000   $ 2,000,000  
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
Verikai, Inc. | Contingent consideration — acquisitions          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Contingent consideration — acquisitions 0        
Gain on change in fair value of contingent consideration liability $ 22,000,000