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Related-Party Transactions - Alliance Agreement (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Related Party Transaction [Line Items]          
Commission revenue $ 157,688 $ 135,789 $ 433,390 $ 370,672  
Markel | Alliance Agreement | Affiliated Entity          
Related Party Transaction [Line Items]          
Due to insurer 108,039   108,039   $ 75,922
Commission revenue $ 105,461 $ 95,559 $ 305,105 $ 267,935  
Markel | Alliance Agreement | Affiliated Entity | Due To Related Parties | Related Party Concentration Risk          
Related Party Transaction [Line Items]          
Percent of total     91.00%   95.00%
Markel | Alliance Agreement | Affiliated Entity | Revenue from Contract with Customer Benchmark | Related Party Concentration Risk          
Related Party Transaction [Line Items]          
Percent of total 92.00% 94.00% 93.00% 95.00%