Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting
Net premiums earned	$ 1,526,406	$ 1,294,306	$ 1,144,436
Casualty Segment
Segment Reporting
Net premiums earned	852,837	758,346	711,832
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	354,847	286,178	253,921
Casualty Segment | Commercial transportation
Segment Reporting
Net premiums earned	120,650	103,719	96,992
Casualty Segment | General liability
Segment Reporting
Net premiums earned	104,423	103,066	100,374
Casualty Segment | Professional services
Segment Reporting
Net premiums earned	103,794	99,596	95,187
Casualty Segment | Small commercial
Segment Reporting
Net premiums earned	78,308	72,920	67,673
Casualty Segment | Executive products
Segment Reporting
Net premiums earned	23,555	24,687	26,606
Casualty Segment | Other casualty
Segment Reporting
Net premiums earned	67,260	68,180	71,079
Property Segment
Segment Reporting
Net premiums earned	531,384	401,530	307,886
Property Segment | Commercial property
Segment Reporting
Net premiums earned	345,554	244,798	163,078
Property Segment | Marine
Segment Reporting
Net premiums earned	145,706	129,428	113,208
Property Segment | Other property
Segment Reporting
Net premiums earned	40,124	27,304	31,600
Surety Segment
Segment Reporting
Net premiums earned	142,185	134,430	124,718
Surety Segment | Transactional
Segment Reporting
Net premiums earned	49,460	47,983	45,826
Surety Segment | Commercial
Segment Reporting
Net premiums earned	48,533	49,707	47,652
Surety Segment | Contract
Segment Reporting
Net premiums earned	$ 44,192	$ 36,740	$ 31,240