Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting
Net premiums earned	$ 360,676	$ 307,723
Casualty Segment
Segment Reporting
Net premiums earned	198,276	186,031
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	80,035	67,582
Casualty Segment | Commercial transportation
Segment Reporting
Net premiums earned	27,301	25,232
Casualty Segment | General liability
Segment Reporting
Net premiums earned	25,412	25,900
Casualty Segment | Professional services
Segment Reporting
Net premiums earned	25,085	24,357
Casualty Segment | Small commercial
Segment Reporting
Net premiums earned	18,337	17,941
Casualty Segment | Executive products
Segment Reporting
Net premiums earned	5,915	6,353
Casualty Segment | Other casualty
Segment Reporting
Net premiums earned	16,191	18,666
Property Segment
Segment Reporting
Net premiums earned	129,411	88,767
Property Segment | Commercial property
Segment Reporting
Net premiums earned	87,605	49,262
Property Segment | Marine
Segment Reporting
Net premiums earned	32,568	30,636
Property Segment | Other property
Segment Reporting
Net premiums earned	9,238	8,869
Surety Segment
Segment Reporting
Net premiums earned	32,989	32,925
Surety Segment | Transactional
Segment Reporting
Net premiums earned	12,108	12,047
Surety Segment | Commercial
Segment Reporting
Net premiums earned	10,625	12,418
Surety Segment | Contract
Segment Reporting
Net premiums earned	$ 10,256	$ 8,460