Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 1,294,306	$ 1,144,436	$ 980,903
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	758,346	711,832	633,639
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	286,178	253,921	219,437
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	103,719	96,992	83,352
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	103,066	100,374	90,853
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	99,596	95,187	88,855
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	72,920	67,673	64,660
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	24,687	26,606	21,873
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	68,180	71,079	64,609
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	401,530	307,886	231,837
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	244,798	163,078	107,941
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	129,428	113,208	97,745
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	27,304	31,600	26,151
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	134,430	124,718	115,427
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	49,707	47,652	43,738
Surety Segment | Transactional
Segment Reporting Information [Line Items]
Net premiums earned	47,983	45,826	43,982
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 36,740	$ 31,240	$ 27,707