Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting
Net premiums earned	$ 379,065	$ 322,280	$ 739,741	$ 630,003
Casualty Segment
Segment Reporting
Net premiums earned	209,100	187,048	407,376	373,079
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	85,986	69,261	166,021	136,843
Casualty Segment | Commercial transportation
Segment Reporting
Net premiums earned	29,323	25,398	56,624	50,630
Casualty Segment | General liability
Segment Reporting
Net premiums earned	26,104	26,016	51,516	51,916
Casualty Segment | Professional services
Segment Reporting
Net premiums earned	26,000	24,602	51,085	48,959
Casualty Segment | Small commercial
Segment Reporting
Net premiums earned	19,459	18,455	37,796	36,396
Casualty Segment | Executive products
Segment Reporting
Net premiums earned	5,448	6,153	11,363	12,506
Casualty Segment | Other casualty
Segment Reporting
Net premiums earned	16,780	17,163	32,971	35,829
Property Segment
Segment Reporting
Net premiums earned	134,097	101,841	263,508	190,608
Property Segment | Commercial property
Segment Reporting
Net premiums earned	87,400	60,219	175,005	109,481
Property Segment | Marine
Segment Reporting
Net premiums earned	37,069	32,412	69,637	63,048
Property Segment | Other property
Segment Reporting
Net premiums earned	9,628	9,210	18,866	18,079
Surety Segment
Segment Reporting
Net premiums earned	35,868	33,391	68,857	66,316
Surety Segment | Transactional
Segment Reporting
Net premiums earned	12,308	11,887	24,416	23,934
Surety Segment | Commercial
Segment Reporting
Net premiums earned	12,700	12,505	23,325	24,923
Surety Segment | Contract
Segment Reporting
Net premiums earned	$ 10,860	$ 8,999	$ 21,116	$ 17,459