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Segment Information - Revenue by Segment (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Segment Reporting Information [Line Items]    
Net premiums earned: $ 1,050,944 $ 902,336
Miscellaneous income 7,801 2,634
Net investment income earned 107,849 91,506
Net realized and unrealized investment gains (losses) (1,635) 3,344
Total Investments revenue 106,214 94,850
Total revenues 1,164,959 999,820
Standard Commercial Lines    
Segment Reporting Information [Line Items]    
Total revenues 841,244 733,796
Standard Commercial Lines | General liability    
Segment Reporting Information [Line Items]    
Net premiums earned: 273,415 243,349
Standard Commercial Lines | Commercial automobile    
Segment Reporting Information [Line Items]    
Net premiums earned: 251,720 217,371
Standard Commercial Lines | Commercial property    
Segment Reporting Information [Line Items]    
Net premiums earned: 161,553 135,292
Standard Commercial Lines | Workers compensation    
Segment Reporting Information [Line Items]    
Net premiums earned: 87,777 84,184
Standard Commercial Lines | Businessowners' policies    
Segment Reporting Information [Line Items]    
Net premiums earned: 39,921 33,171
Standard Commercial Lines | Bonds    
Segment Reporting Information [Line Items]    
Net premiums earned: 12,088 11,397
Standard Commercial Lines | Other    
Segment Reporting Information [Line Items]    
Net premiums earned: 7,636 6,851
Standard Commercial Lines | Miscellaneous income    
Segment Reporting Information [Line Items]    
Miscellaneous income 7,134 2,181
Standard Personal Lines    
Segment Reporting Information [Line Items]    
Total revenues 104,486 82,323
Standard Personal Lines | Miscellaneous income    
Segment Reporting Information [Line Items]    
Miscellaneous income 640 453
Standard Personal Lines | Personal automobile    
Segment Reporting Information [Line Items]    
Net premiums earned: 56,960 44,914
Standard Personal Lines | Homeowners    
Segment Reporting Information [Line Items]    
Net premiums earned: 44,113 35,013
Standard Personal Lines | Other    
Segment Reporting Information [Line Items]    
Net premiums earned: 2,773 1,943
E&S Lines    
Segment Reporting Information [Line Items]    
Total revenues 113,015 88,851
E&S Lines | Miscellaneous income    
Segment Reporting Information [Line Items]    
Miscellaneous income 27 0
E&S Lines | Casualty lines    
Segment Reporting Information [Line Items]    
Net premiums earned: 71,638 60,817
E&S Lines | Property lines    
Segment Reporting Information [Line Items]    
Net premiums earned: $ 41,350 $ 28,034