Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Segment Reporting
Net premiums earned	$ 417,096	$ 401,904	$ 828,482	$ 800,249
Casualty Segment
Segment Reporting
Net premiums earned	256,922	234,638	505,488	463,686
Casualty Segment \| Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	126,854	106,055	248,125	206,530
Casualty Segment \| Commercial transportation
Segment Reporting
Net premiums earned	33,632	30,015	64,326	60,274
Casualty Segment \| General liability
Segment Reporting
Net premiums earned	22,850	23,869	45,658	46,799
Casualty Segment \| Professional services
Segment Reporting
Net premiums earned	28,134	26,623	56,083	53,210
Casualty Segment \| Small commercial
Segment Reporting
Net premiums earned	19,886	20,000	39,208	39,915
Casualty Segment \| Executive products
Segment Reporting
Net premiums earned	5,481	5,521	11,354	11,464
Casualty Segment \| Other casualty
Segment Reporting
Net premiums earned	20,085	22,555	40,734	45,494
Property Segment
Segment Reporting
Net premiums earned	123,786	130,664	250,164	263,208
Property Segment \| Commercial property
Segment Reporting
Net premiums earned	68,900	76,876	141,860	159,688
Property Segment \| Marine
Segment Reporting
Net premiums earned	40,337	41,017	79,556	78,726
Property Segment \| Other property
Segment Reporting
Net premiums earned	14,549	12,771	28,748	24,794
Surety Segment
Segment Reporting
Net premiums earned	36,388	36,602	72,830	73,355
Surety Segment \| Transactional
Segment Reporting
Net premiums earned	13,347	13,096	26,642	25,756
Surety Segment \| Commercial
Segment Reporting
Net premiums earned	12,402	12,422	25,037	25,198
Surety Segment \| Contract
Segment Reporting
Net premiums earned	$ 10,639	$ 11,084	$ 21,151	$ 22,401