v3.26.1
Segment Information - Schedule of Revenue by Segment (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
Net investment income earned 150,167 127,968 292,550 248,659
Net realized and unrealized investment gains (losses) 11,971 4,172 3,670 4,401
Other income 9,389 6,548 17,036 12,057
Total revenues 1,387,035 1,326,745 2,745,960 2,611,931
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
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
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
Operating Segments        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total revenues 1,377,646 1,320,197 2,728,924 2,599,874
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
Total revenues 962,044 937,635 1,927,803 1,849,845
Operating Segments | Standard Commercial Lines | General liability        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 318,688 305,843 633,790 600,530
Operating Segments | Standard Commercial Lines | Commercial automobile        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 291,411 288,759 587,791 572,344
Operating Segments | Standard Commercial Lines | Commercial property        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 198,475 191,027 400,109 377,557
Operating Segments | Standard Commercial Lines | Workers compensation        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 81,906 82,024 161,727 161,060
Operating Segments | Standard Commercial Lines | Businessowners' policies        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 52,443 48,416 104,202 95,309
Operating Segments | Standard Commercial Lines | Bonds        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 9,921 13,255 22,008 26,513
Operating Segments | Standard Commercial Lines | Other        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 9,200 8,311 18,176 16,532
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
Total revenues 97,639 102,377 197,667 206,032
Operating Segments | Standard Personal Lines | Personal automobile        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 44,656 51,287 91,320 104,255
Operating Segments | Standard Personal Lines | Homeowners        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 49,937 48,312 100,016 96,255
Operating Segments | Standard Personal Lines | Other        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 3,046 2,778 6,331 5,522
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
Total revenues 155,825 148,045 307,234 290,937
Operating Segments | E&S Lines | Casualty lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 92,663 87,400 182,174 172,519
Operating Segments | E&S Lines | Property lines        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net premiums earned 63,162 60,645 125,060 118,418
Operating Segments | Investments        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Net investment income earned 150,167 127,968 292,550 248,659
Net realized and unrealized investment gains (losses) 11,971 4,172 3,670 4,401
Total revenues 162,138 132,140 296,220 253,060
Other        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Other income $ 9,389 $ 6,548 $ 17,036 $ 12,057