Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting
Net premiums earned	$ 407,695	$ 389,489	$ 1,207,944	$ 1,129,230
Casualty Segment
Segment Reporting
Net premiums earned	243,508	219,638	707,194	627,014
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting
Net premiums earned	115,132	91,434	327,252	257,455
Casualty Segment | Commercial transportation
Segment Reporting
Net premiums earned	31,190	32,067	91,464	88,691
Casualty Segment | General liability
Segment Reporting
Net premiums earned	28,690	26,316	82,997	77,832
Casualty Segment | Professional services
Segment Reporting
Net premiums earned	27,431	26,281	80,641	77,366
Casualty Segment | Small commercial
Segment Reporting
Net premiums earned	19,627	20,074	59,542	57,870
Casualty Segment | Executive products
Segment Reporting
Net premiums earned	6,043	6,261	17,507	17,624
Casualty Segment | Other casualty
Segment Reporting
Net premiums earned	15,395	17,205	47,791	50,176
Property Segment
Segment Reporting
Net premiums earned	126,689	133,266	389,897	396,774
Property Segment | Commercial property
Segment Reporting
Net premiums earned	72,853	85,814	232,541	260,819
Property Segment | Marine
Segment Reporting
Net premiums earned	40,058	37,618	118,784	107,255
Property Segment | Other property
Segment Reporting
Net premiums earned	13,778	9,834	38,572	28,700
Surety Segment
Segment Reporting
Net premiums earned	37,498	36,585	110,853	105,442
Surety Segment | Transactional
Segment Reporting
Net premiums earned	13,268	12,488	39,024	36,904
Surety Segment | Commercial
Segment Reporting
Net premiums earned	12,700	12,477	37,898	35,802
Surety Segment | Contract
Segment Reporting
Net premiums earned	$ 11,530	$ 11,620	$ 33,931	$ 32,736