v3.26.3
Segment Information - Schedule of Combined Ratio (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned $ 1,180,191 $ 1,204,678 $ 3,612,895 $ 3,551,492
Loss and loss expense incurred 762,696 819,031 2,394,466 2,389,254
Income (loss) before income tax 162,088 145,711 449,135 393,502
Insurance Operations        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 1,180,191 1,204,678 3,612,895 3,551,492
Loss and loss expense incurred 762,696 819,031 2,394,466 2,389,254
Net underwriting expenses incurred 379,197 368,073 1,133,307 1,099,316
Dividends to policyholders $ 913 $ 876 $ 2,103 $ 3,010
Ratio        
Loss and loss expense incurred (as a percent) 64.60% 67.90% 66.20% 67.20%
Net underwriting expenses incurred (as a percent) 32.10% 30.60% 31.40% 31.00%
Dividends to policyholders (as a percent) 0.10% 0.10% 0.10% 0.10%
Underwriting income (loss) (as a percent) 96.80% 98.60% 97.70% 98.30%
Standard Commercial Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned $ 927,104 $ 947,272 $ 2,854,907 $ 2,797,117
Loss and loss expense incurred 600,255 657,958 1,909,338 1,906,390
Net underwriting expenses incurred 306,786 298,614 918,912 891,119
Dividends to policyholders $ 913 $ 876 $ 2,103 $ 3,010
Ratio        
Loss and loss expense incurred (as a percent) 64.70% 69.50% 66.80% 68.10%
Net underwriting expenses incurred (as a percent) 33.10% 31.50% 32.20% 31.90%
Dividends to policyholders (as a percent) 0.10% 0.10% 0.10% 0.10%
Underwriting income (loss) (as a percent) 97.90% 101.10% 99.10% 100.10%
Standard Personal Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned $ 95,325 $ 101,519 $ 292,992 $ 307,551
Loss and loss expense incurred 73,507 89,290 211,810 235,936
Net underwriting expenses incurred $ 24,866 $ 22,472 $ 72,644 $ 71,271
Ratio        
Loss and loss expense incurred (as a percent) 77.10% 88.00% 72.30% 76.70%
Net underwriting expenses incurred (as a percent) 26.10% 22.10% 24.80% 23.20%
Underwriting income (loss) (as a percent) 103.20% 110.10% 97.10% 99.90%
E&S Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned $ 157,762 $ 155,887 $ 464,996 $ 446,824
Loss and loss expense incurred 88,934 71,783 273,318 246,928
Net underwriting expenses incurred $ 47,545 $ 46,987 $ 141,751 $ 136,926
Ratio        
Loss and loss expense incurred (as a percent) 56.40% 46.10% 58.80% 55.30%
Net underwriting expenses incurred (as a percent) 30.10% 30.10% 30.50% 30.60%
Underwriting income (loss) (as a percent) 86.50% 76.20% 89.30% 85.90%
Operating Segments        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Loss and loss expense incurred $ 762,696 $ 819,031 $ 2,394,466 $ 2,389,254
Dividends to policyholders 913 876 2,103 3,010
Income (loss) before income tax 178,420 163,410 520,274 459,684
Operating Segments | Insurance Operations        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Loss and loss expense incurred 762,696 819,031 2,394,466 2,389,254
Dividends to policyholders 913 876 2,103 3,010
Income (loss) before income tax 37,385 16,698 83,019 59,912
Operating Segments | Standard Commercial Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 927,104 947,272 2,854,907 2,797,117
Loss and loss expense incurred 600,255 657,958 1,909,338 1,906,390
Dividends to policyholders 913 876 2,103 3,010
Income (loss) before income tax 19,150 (10,176) 24,554 (3,402)
Operating Segments | Standard Personal Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 95,325 101,519 292,992 307,551
Loss and loss expense incurred 73,507 89,290 211,810 235,936
Dividends to policyholders 0 0 0 0
Income (loss) before income tax (3,048) (10,243) 8,538 344
Operating Segments | E&S Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 157,762 155,887 464,996 446,824
Loss and loss expense incurred 88,934 71,783 273,318 246,928
Dividends to policyholders 0 0 0 0
Income (loss) before income tax $ 21,283 $ 37,117 $ 49,927 $ 62,970