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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

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