Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 318,409	$ 291,468	$ 948,412	$ 843,430
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	189,305	179,615	562,384	528,494
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	72,559	65,131	209,402	187,666
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	25,250	25,284	77,166	74,130
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	25,843	24,205	76,473	71,774
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	24,882	24,174	73,841	71,590
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	18,096	17,005	54,492	50,463
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	5,943	6,556	18,449	20,208
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	16,732	17,260	52,561	52,663
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	94,988	80,844	285,596	222,974
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	63,783	43,201	173,264	115,419
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	33,436	29,592	96,484	84,499
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	(2,231)	8,051	15,848	23,056
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	34,116	31,009	100,432	91,962
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	12,653	12,057	37,576	35,423
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	11,966	10,751	35,900	33,691
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 9,497	$ 8,201	$ 26,956	$ 22,848