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Segment Information - Schedule of Revenue by Segment (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Revenue from External Customer [Line Items]        
Net premiums earned $ 1,204,678 $ 1,112,228 $ 3,551,492 $ 3,243,403
Miscellaneous income 8,720 8,930 20,777 22,566
Net investment income earned 138,655 117,759 387,314 334,250
Net realized and unrealized investment gains (losses) 8,057 5,389 12,458 5,051
Total Investments revenue 146,712 123,148 399,772 339,301
Total revenues 1,360,110 1,244,306 3,972,041 3,605,270
Standard Commercial Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 947,272 875,379 2,797,117 2,562,982
Total revenues 955,424 883,568 2,816,049 2,583,519
Standard Personal Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 101,519 107,521 307,551 317,788
Total revenues 102,053 108,213 309,233 319,715
E&S Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 155,887 129,328 446,824 362,633
Total revenues 155,921 129,377 446,987 362,735
General liability | Standard Commercial Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 312,556 286,641 913,086 840,153
Commercial automobile | Standard Commercial Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 294,014 269,036 866,358 781,408
Commercial property | Standard Commercial Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 194,037 174,855 571,594 504,919
Workers compensation | Standard Commercial Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 75,528 81,296 236,588 251,389
Businessowners' policies | Standard Commercial Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 49,673 43,091 144,982 124,653
Bonds | Standard Commercial Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 13,046 12,511 39,559 37,067
Other | Standard Commercial Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 8,418 7,949 24,950 23,393
Miscellaneous income | Standard Commercial Lines        
Revenue from External Customer [Line Items]        
Miscellaneous income 8,152 8,189 18,932 20,537
Miscellaneous income | Standard Personal Lines        
Revenue from External Customer [Line Items]        
Miscellaneous income 534 692 1,682 1,927
Miscellaneous income | E&S Lines        
Revenue from External Customer [Line Items]        
Miscellaneous income 34 49 163 102
Personal automobile | Standard Personal Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 49,933 56,599 154,188 171,103
Homeowners | Standard Personal Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 48,742 47,251 144,997 137,419
Other | Standard Personal Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 2,844 3,671 8,366 9,266
Casualty lines | E&S Lines        
Revenue from External Customer [Line Items]        
Net premiums earned 92,771 77,471 265,290 222,996
Property lines | E&S Lines        
Revenue from External Customer [Line Items]        
Net premiums earned $ 63,116 $ 51,857 $ 181,534 $ 139,637