Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 980,903	$ 865,747	$ 839,111
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	633,639	569,521	558,458
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	219,437	178,214	140,483
Casualty Segment | General Liability
Segment Reporting Information [Line Items]
Net premiums earned	90,853	91,653	98,880
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	88,855	85,196	81,329
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	83,352	64,624	83,213
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	64,660	63,357	55,701
Casualty Segment | Executive Products
Segment Reporting Information [Line Items]
Net premiums earned	21,873	26,509	27,088
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	64,609	59,968	71,764
Property segment
Segment Reporting Information [Line Items]
Net premiums earned	231,837	183,720	164,022
Property segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	97,745	81,852	74,887
Property segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	107,941	79,406	68,310
Property segment | Specialty Personal
Segment Reporting Information [Line Items]
Net premiums earned	21,385	19,596	19,316
Property segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	4,766	2,866	1,509
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	115,427	112,506	116,631
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	43,738	42,872	43,553
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	43,982	42,292	44,721
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 27,707	$ 27,342	$ 28,357