Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 269,152	$ 228,595
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	171,756	148,770
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	60,072	51,554
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	23,740	22,407
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	23,628	16,830
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	23,555	21,728
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	16,645	15,722
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	6,577	5,241
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	17,539	15,288
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	67,440	51,642
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	33,289	22,712
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	26,729	22,958
Property Segment | Specialty personal
Segment Reporting Information [Line Items]
Net premiums earned	5,896	5,034
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	1,526	938
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	29,956	28,183
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	11,703	11,013
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	11,353	10,635
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 6,900	$ 6,535