Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting
Net premiums earned	$ 411,386	$ 398,345
Casualty Segment
Segment Reporting
Net premiums earned	248,566	229,048
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	124,216	103,209
Casualty Segment | Commercial transportation
Segment Reporting
Net premiums earned	30,694	30,259
Casualty Segment | General liability
Segment Reporting
Net premiums earned	26,208	26,718
Casualty Segment | Professional services
Segment Reporting
Net premiums earned	27,949	26,587
Casualty Segment | Small commercial
Segment Reporting
Net premiums earned	19,322	19,915
Casualty Segment | Executive products
Segment Reporting
Net premiums earned	5,873	5,943
Casualty Segment | Other casualty
Segment Reporting
Net premiums earned	14,304	16,417
Property Segment
Segment Reporting
Net premiums earned	126,378	132,544
Property Segment | Commercial property
Segment Reporting
Net premiums earned	72,960	82,812
Property Segment | Marine
Segment Reporting
Net premiums earned	39,219	37,709
Property Segment | Other property
Segment Reporting
Net premiums earned	14,199	12,023
Surety Segment
Segment Reporting
Net premiums earned	36,442	36,753
Surety Segment | Transactional
Segment Reporting
Net premiums earned	13,295	12,660
Surety Segment | Commercial
Segment Reporting
Net premiums earned	12,635	12,776
Surety Segment | Contract
Segment Reporting
Net premiums earned	$ 10,512	$ 11,317