Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 322,280	$ 282,810	$ 630,003	$ 551,962
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	187,048	177,123	373,079	348,879
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	69,261	62,463	136,843	122,535
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	26,016	25,106	51,916	48,846
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	25,398	23,941	50,630	47,569
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	24,602	23,861	48,959	47,416
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	18,455	16,813	36,396	33,458
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	6,153	7,075	12,506	13,652
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	17,163	17,864	35,829	35,403
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	101,841	74,690	190,608	142,130
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	60,219	38,929	109,481	72,218
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	32,412	28,178	63,048	54,907
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	9,210	7,583	18,079	15,005
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	33,391	30,997	66,316	60,953
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	12,505	11,663	24,923	23,366
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	11,887	11,587	23,934	22,940
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 8,999	$ 7,747	$ 17,459	$ 14,647