Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Net premiums earned	$ 253,389	$ 216,630	$ 722,984	$ 640,946
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	162,852	143,002	467,817	421,637
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	56,285	46,404	161,287	128,813
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	23,156	22,747	67,616	69,396
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	22,401	21,422	66,095	63,501
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	22,571	15,130	60,926	47,454
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	16,301	16,009	48,302	47,433
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	5,461	6,387	15,785	20,693
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	16,677	14,903	47,806	44,347
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	60,886	45,380	168,393	135,115
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	25,416	19,687	72,737	59,976
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	28,683	20,168	76,488	58,371
Property Segment | Specialty personal
Segment Reporting Information [Line Items]
Net premiums earned	5,462	4,794	15,761	14,730
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	1,325	731	3,407	2,038
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	29,651	28,248	86,774	84,194
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	11,541	10,543	33,594	32,287
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	11,124	10,547	32,604	31,562
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 6,986	$ 7,158	$ 20,576	$ 20,345