Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Net premiums earned	$ 241,000	$ 208,734	$ 469,595	$ 424,316
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	156,195	135,215	304,965	278,635
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	53,448	42,321	105,002	82,409
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	22,053	22,651	44,460	46,649
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	21,966	21,384	43,694	42,079
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	21,525	11,139	38,355	32,324
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	16,279	15,791	32,001	31,424
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	5,083	6,975	10,324	14,306
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	15,841	14,954	31,129	29,444
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	55,865	45,387	107,507	89,735
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	24,363	20,712	47,321	40,289
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	25,093	19,048	47,805	38,203
Property Segment | Specialty personal
Segment Reporting Information [Line Items]
Net premiums earned	5,264	4,936	10,298	9,936
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	1,145	691	2,083	1,307
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	28,940	28,132	57,123	55,946
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	11,040	10,806	22,053	21,744
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	10,845	10,499	21,480	21,015
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 7,055	$ 6,827	$ 13,590	$ 13,187