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Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2026 | Mar. 31, 2025
Segment Reporting |  |
Net premiums earned | $ 411,386 | $ 398,345
Casualty Segment |  |
Segment Reporting |  |
Net premiums earned | 248,566 | 229,048
Casualty Segment | Commercial excess and personal umbrella |  |
Segment Reporting |  |
Net premiums earned | 124,216 | 103,209
Casualty Segment | Commercial transportation |  |
Segment Reporting |  |
Net premiums earned | 30,694 | 30,259
Casualty Segment | General liability |  |
Segment Reporting |  |
Net premiums earned | 26,208 | 26,718
Casualty Segment | Professional services |  |
Segment Reporting |  |
Net premiums earned | 27,949 | 26,587
Casualty Segment | Small commercial |  |
Segment Reporting |  |
Net premiums earned | 19,322 | 19,915
Casualty Segment | Executive products |  |
Segment Reporting |  |
Net premiums earned | 5,873 | 5,943
Casualty Segment | Other casualty |  |
Segment Reporting |  |
Net premiums earned | 14,304 | 16,417
Property Segment |  |
Segment Reporting |  |
Net premiums earned | 126,378 | 132,544
Property Segment | Commercial property |  |
Segment Reporting |  |
Net premiums earned | 72,960 | 82,812
Property Segment | Marine |  |
Segment Reporting |  |
Net premiums earned | 39,219 | 37,709
Property Segment | Other property |  |
Segment Reporting |  |
Net premiums earned | 14,199 | 12,023
Surety Segment |  |
Segment Reporting |  |
Net premiums earned | 36,442 | 36,753
Surety Segment | Transactional |  |
Segment Reporting |  |
Net premiums earned | 13,295 | 12,660
Surety Segment | Commercial |  |
Segment Reporting |  |
Net premiums earned | 12,635 | 12,776
Surety Segment | Contract |  |
Segment Reporting |  |
Net premiums earned | $ 10,512 | $ 11,317

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