v3.26.3
Segment Information - Schedule of Revenue by Segment (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
Net investment income earned 160,481 138,655 453,031 387,314
Net realized and unrealized investment gains (losses) (19,446) 8,057 (15,776) 12,458
Other income 5,074 8,720 22,110 20,777
Total revenues 1,326,300 1,360,110 4,072,260 3,972,041
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
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
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
Operating Segments        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Total revenues 1,321,226 1,351,390 4,050,150 3,951,264
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
Total revenues 927,104 947,272 2,854,907 2,797,117
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 302,146 312,556 935,936 913,086
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 281,698 294,014 869,489 866,358
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 197,020 194,037 597,129 571,594
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 73,583 75,528 235,310 236,588
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 52,646 49,673 156,848 144,982
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 10,595 13,046 32,603 39,559
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 9,416 8,418 27,592 24,950
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
Total revenues 95,325 101,519 292,992 307,551
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 42,533 49,933 133,853 154,188
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 49,861 48,742 149,877 144,997
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 2,931 2,844 9,262 8,366
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
Total revenues 157,762 155,887 464,996 446,824
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 93,826 92,771 276,000 265,290
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 63,936 63,116 188,996 181,534
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 160,481 138,655 453,031 387,314
Net realized and unrealized investment gains (losses) (19,446) 8,057 (15,776) 12,458
Total revenues 141,035 146,712 437,255 399,772
Other        
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Product and Service, Revenue [Line Items]        
Other income $ 5,074 $ 8,720 $ 22,110 $ 20,777