Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Net premiums earned	$ 228,595	$ 215,582
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	148,770	143,420
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	51,554	40,088
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	22,407	23,998
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	21,728	20,695
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	16,830	21,185
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	15,722	15,633
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	5,241	7,331
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	15,288	14,490
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	51,642	44,348
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	22,958	19,577
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	22,712	19,155
Property Segment | Specialty personal
Segment Reporting Information [Line Items]
Net premiums earned	5,034	5,000
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	938	616
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	28,183	27,814
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	11,013	10,938
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	10,635	10,516
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 6,535	$ 6,360