Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 216,630	$ 211,255	$ 640,946	$ 623,485
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	143,002	140,423	421,637	415,667
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	46,404	35,959	128,813	102,580
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	22,747	25,005	69,396	74,240
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	21,422	20,816	63,501	60,390
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	15,130	21,067	47,454	61,979
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	16,009	13,927	47,433	40,430
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	6,387	7,026	20,693	19,476
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	14,903	16,623	44,347	56,572
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	45,380	41,476	135,115	120,194
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	19,687	19,033	59,976	54,633
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	20,168	17,098	58,371	50,248
Property Segment | Specialty personal
Segment Reporting Information [Line Items]
Net premiums earned	4,794	4,948	14,730	14,324
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	731	397	2,038	989
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	28,248	29,356	84,194	87,624
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	10,543	10,851	32,287	32,638
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	10,547	10,998	31,562	33,880
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 7,158	$ 7,507	$ 20,345	$ 21,106