Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting
Net premiums earned	$ 424,914	$ 407,695	$ 1,253,396	$ 1,207,944
Casualty Segment
Segment Reporting
Net premiums earned	262,177	243,508	767,665	707,194
Casualty Segment \| Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	132,606	112,175	380,731	318,705
Casualty Segment \| Commercial transportation
Segment Reporting
Net premiums earned	33,508	31,190	97,834	91,464
Casualty Segment \| General liability
Segment Reporting
Net premiums earned	22,524	25,007	68,182	71,806
Casualty Segment \| Professional services
Segment Reporting
Net premiums earned	28,168	27,431	84,251	80,641
Casualty Segment \| Small commercial
Segment Reporting
Net premiums earned	19,587	19,627	58,795	59,542
Casualty Segment \| Executive products
Segment Reporting
Net premiums earned	5,646	6,043	17,000	17,507
Casualty Segment \| Other casualty
Segment Reporting
Net premiums earned	20,138	22,035	60,872	67,529
Property Segment
Segment Reporting
Net premiums earned	126,520	126,689	376,684	389,897
Property Segment \| Commercial property
Segment Reporting
Net premiums earned	66,521	72,853	208,381	232,541
Property Segment \| Marine
Segment Reporting
Net premiums earned	43,412	40,058	122,968	118,784
Property Segment \| Other property
Segment Reporting
Net premiums earned	16,587	13,778	45,335	38,572
Surety Segment
Segment Reporting
Net premiums earned	36,217	37,498	109,047	110,853
Surety Segment \| Transactional
Segment Reporting
Net premiums earned	13,737	13,268	40,379	39,024
Surety Segment \| Commercial
Segment Reporting
Net premiums earned	11,942	12,700	36,979	37,898
Surety Segment \| Contract
Segment Reporting
Net premiums earned	$ 10,538	$ 11,530	$ 31,689	$ 33,931