Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting
Net premiums earned	$ 401,904	$ 379,065	$ 800,249	$ 739,741
Casualty Segment
Segment Reporting
Net premiums earned	234,638	209,100	463,686	407,376
Casualty Segment \| Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	108,911	85,986	212,120	166,021
Casualty Segment \| Commercial transportation
Segment Reporting
Net premiums earned	30,015	29,323	60,274	56,624
Casualty Segment \| General liability
Segment Reporting
Net premiums earned	27,589	26,104	54,307	51,516
Casualty Segment \| Professional services
Segment Reporting
Net premiums earned	26,623	26,000	53,210	51,085
Casualty Segment \| Small commercial
Segment Reporting
Net premiums earned	20,000	19,459	39,915	37,796
Casualty Segment \| Executive products
Segment Reporting
Net premiums earned	5,521	5,448	11,464	11,363
Casualty Segment \| Other casualty
Segment Reporting
Net premiums earned	15,979	16,780	32,396	32,971
Property Segment
Segment Reporting
Net premiums earned	130,664	134,097	263,208	263,508
Property Segment \| Commercial property
Segment Reporting
Net premiums earned	76,876	87,400	159,688	175,005
Property Segment \| Marine
Segment Reporting
Net premiums earned	41,017	37,069	78,726	69,637
Property Segment \| Other property
Segment Reporting
Net premiums earned	12,771	9,628	24,794	18,866
Surety Segment
Segment Reporting
Net premiums earned	36,602	35,868	73,355	68,857
Surety Segment \| Commercial
Segment Reporting
Net premiums earned	12,422	12,700	25,198	23,325
Surety Segment \| Transactional
Segment Reporting
Net premiums earned	13,096	12,308	25,756	24,416
Surety Segment \| Contract
Segment Reporting
Net premiums earned	$ 11,084	$ 10,860	$ 22,401	$ 21,116