Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting
Net premiums earned	$ 398,345	$ 360,676
Casualty Segment
Segment Reporting
Net premiums earned	229,048	198,276
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	103,209	80,035
Casualty Segment | Commercial transportation
Segment Reporting
Net premiums earned	30,259	27,301
Casualty Segment | General liability
Segment Reporting
Net premiums earned	26,718	25,412
Casualty Segment | Professional services
Segment Reporting
Net premiums earned	26,587	25,085
Casualty Segment | Small commercial
Segment Reporting
Net premiums earned	19,915	18,337
Casualty Segment | Executive products
Segment Reporting
Net premiums earned	5,943	5,915
Casualty Segment | Other casualty
Segment Reporting
Net premiums earned	16,417	16,191
Property Segment
Segment Reporting
Net premiums earned	132,544	129,411
Property Segment | Commercial property
Segment Reporting
Net premiums earned	82,812	87,605
Property Segment | Marine
Segment Reporting
Net premiums earned	37,709	32,568
Property Segment | Other property
Segment Reporting
Net premiums earned	12,023	9,238
Surety Segment
Segment Reporting
Net premiums earned	36,753	32,989
Surety Segment | Commercial
Segment Reporting
Net premiums earned	12,776	10,625
Surety Segment | Transactional
Segment Reporting
Net premiums earned	12,660	12,108
Surety Segment | Contract
Segment Reporting
Net premiums earned	$ 11,317	$ 10,256