v3.26.1
Segment Information - Schedule of Combined Ratio (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned $ 1,215,508 $ 1,188,057 $ 2,432,704 $ 2,346,814
Loss and loss expense incurred 816,266 823,898 1,631,770 1,570,223
Income (loss) before income tax 162,845 108,905 287,047 247,791
Insurance Operations        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 1,215,508 1,188,057 2,432,704 2,346,814
Loss and loss expense incurred 816,266 823,898 1,631,770 1,570,223
Net underwriting expenses incurred 374,363 365,431 754,110 731,243
Dividends to policyholders $ 490 $ 1,151 $ 1,190 $ 2,134
Ratio        
Loss and loss expense incurred (as a percent) 67.20% 69.30% 67.10% 66.90%
Net underwriting expenses incurred (as a percent) 30.80% 30.80% 31.00% 31.20%
Dividends to policyholders (as a percent) 0.00% 0.10% 0.00% 0.10%
Underwriting income (loss) (as a percent) 98.00% 100.20% 98.10% 98.20%
Standard Commercial Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned $ 962,044 $ 937,635 $ 1,927,803 $ 1,849,845
Loss and loss expense incurred 652,249 666,766 1,309,083 1,248,432
Net underwriting expenses incurred 302,100 295,862 612,127 592,505
Dividends to policyholders $ 490 $ 1,151 $ 1,190 $ 2,134
Ratio        
Loss and loss expense incurred (as a percent) 67.80% 71.10% 67.80% 67.50%
Net underwriting expenses incurred (as a percent) 31.40% 31.60% 31.80% 32.00%
Dividends to policyholders (as a percent) 0.10% 0.10% 0.10% 0.10%
Underwriting income (loss) (as a percent) 99.30% 102.80% 99.70% 99.60%
Standard Personal Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned $ 97,639 $ 102,377 $ 197,667 $ 206,032
Loss and loss expense incurred 69,004 69,977 138,303 146,646
Net underwriting expenses incurred $ 24,207 $ 23,850 $ 47,778 $ 48,799
Ratio        
Loss and loss expense incurred (as a percent) 70.70% 68.30% 69.90% 71.20%
Net underwriting expenses incurred (as a percent) 24.80% 23.30% 24.20% 23.70%
Underwriting income (loss) (as a percent) 95.50% 91.60% 94.10% 94.90%
E&S Lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned $ 155,825 $ 148,045 $ 307,234 $ 290,937
Loss and loss expense incurred 95,013 87,155 184,384 175,145
Net underwriting expenses incurred $ 48,056 $ 45,719 $ 94,205 $ 89,939
Ratio        
Loss and loss expense incurred (as a percent) 61.00% 58.90% 60.00% 60.20%
Net underwriting expenses incurred (as a percent) 30.80% 30.90% 30.70% 30.90%
Underwriting income (loss) (as a percent) 91.80% 89.80% 90.70% 91.10%
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 $ 816,266 $ 823,898 $ 1,631,770 $ 1,570,223
Dividends to policyholders 490 1,151 1,190 2,134
Income (loss) before income tax 186,527 129,717 341,854 296,274
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 816,266 823,898 1,631,770 1,570,223
Dividends to policyholders 490 1,151 1,190 2,134
Income (loss) before income tax 24,389 (2,423) 45,634 43,214
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 962,044 937,635 1,927,803 1,849,845
Loss and loss expense incurred 652,249 666,766 1,309,083 1,248,432
Dividends to policyholders 490 1,151 1,190 2,134
Income (loss) before income tax 7,205 (26,144) 5,403 6,774
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 97,639 102,377 197,667 206,032
Loss and loss expense incurred 69,004 69,977 138,303 146,646
Dividends to policyholders 0 0 0 0
Income (loss) before income tax 4,428 8,550 11,586 10,587
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 155,825 148,045 307,234 290,937
Loss and loss expense incurred 95,013 87,155 184,384 175,145
Dividends to policyholders 0 0 0 0
Income (loss) before income tax $ 12,756 $ 15,171 $ 28,645 $ 25,853