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Segment Information - Schedule of Revenue by Segment (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Revenue from External Customer [Line Items]      
Net premiums earned $ 4,376,447 $ 3,827,606 $ 3,373,380
Miscellaneous income 31,115 19,402 11,335
Net investment income 457,051 388,650 288,155
Net realized and unrealized investment gains (losses) (2,949) (3,552) (114,808)
Total Investments revenues 454,102 385,098 173,347
Total revenues 4,861,664 4,232,106 3,558,062
Standard Commercial Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 3,447,556 3,071,784 2,739,819
Total revenues 3,475,950 3,088,952 2,749,338
Standard Personal Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 424,917 365,213 299,405
Total revenues 427,483 367,414 301,221
E&S Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 503,974 390,609 334,156
Total revenues 504,129 390,642 334,156
General liability | Standard Commercial Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 1,125,491 1,020,362 902,428
Commercial automobile | Standard Commercial Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 1,058,228 916,140 812,306
Commercial property | Standard Commercial Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 685,568 586,267 495,647
Workers compensation | Standard Commercial Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 327,725 333,669 335,955
Businessowners’ policies | Standard Commercial Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 169,321 140,547 124,474
Bonds | Standard Commercial Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 49,748 46,215 43,354
Other | Standard Commercial Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 31,475 28,584 25,655
Miscellaneous income | Standard Commercial Lines      
Revenue from External Customer [Line Items]      
Miscellaneous income 28,394 17,168 9,519
Miscellaneous income | Standard Personal Lines      
Revenue from External Customer [Line Items]      
Miscellaneous income 2,566 2,201 1,816
Miscellaneous income | E&S Lines      
Revenue from External Customer [Line Items]      
Miscellaneous income 155 33 0
Personal automobile | Standard Personal Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 226,138 200,027 162,899
Homeowners | Standard Personal Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 185,651 154,828 128,222
Other | Standard Personal Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 13,128 10,358 8,284
Casualty lines | E&S Lines      
Revenue from External Customer [Line Items]      
Net premiums earned 307,895 261,144 233,086
Property lines | E&S Lines      
Revenue from External Customer [Line Items]      
Net premiums earned $ 196,079 $ 129,465 $ 101,070