Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 291,468	$ 253,389	$ 843,430	$ 722,984
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	179,615	162,852	528,494	467,817
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	65,131	56,285	187,666	161,287
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	25,284	23,156	74,130	67,616
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	24,205	22,571	71,774	60,926
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	24,174	22,401	71,590	66,095
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	17,005	16,301	50,463	48,302
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	6,556	5,461	20,208	15,785
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	17,260	16,677	52,663	47,806
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	80,844	60,886	222,974	168,393
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	43,201	28,683	115,419	76,488
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	29,592	25,416	84,499	72,737
Property Segment | Specialty personal
Segment Reporting Information [Line Items]
Net premiums earned	6,409	5,462	18,405	15,761
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	1,642	1,325	4,651	3,407
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	31,009	29,651	91,962	86,774
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	12,057	11,541	35,423	33,594
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	10,751	11,124	33,691	32,604
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 8,201	$ 6,986	$ 22,848	$ 20,576