Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 282,810	$ 241,000	$ 551,962	$ 469,595
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	177,123	156,195	348,879	304,965
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	62,463	53,448	122,535	105,002
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	25,106	22,053	48,846	44,460
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	23,941	21,525	47,569	38,355
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	23,861	21,966	47,416	43,694
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	16,813	16,279	33,458	32,001
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	7,075	5,083	13,652	10,324
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	17,864	15,841	35,403	31,129
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	74,690	55,865	142,130	107,507
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	38,929	25,093	72,218	47,805
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	28,178	24,363	54,907	47,321
Property Segment | Specialty personal
Segment Reporting Information [Line Items]
Net premiums earned	6,100	5,264	11,996	10,298
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	1,483	1,145	3,009	2,083
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	30,997	28,940	60,953	57,123
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	11,663	11,040	23,366	22,053
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	11,587	10,845	22,940	21,480
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 7,747	$ 7,055	$ 14,647	$ 13,590