Operating Segment Information - Summary of Revenue by Major Product Type (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Net premiums earned	$ 307,723	$ 269,152
Casualty Segment
Segment Reporting Information [Line Items]
Net premiums earned	186,031	171,756
Casualty Segment | Commercial excess and personal umbrella
Segment Reporting Information [Line Items]
Net premiums earned	67,582	60,072
Casualty Segment | General liability
Segment Reporting Information [Line Items]
Net premiums earned	25,900	23,740
Casualty Segment | Commercial transportation
Segment Reporting Information [Line Items]
Net premiums earned	25,232	23,628
Casualty Segment | Professional services
Segment Reporting Information [Line Items]
Net premiums earned	24,357	23,555
Casualty Segment | Small commercial
Segment Reporting Information [Line Items]
Net premiums earned	17,941	16,645
Casualty Segment | Executive products
Segment Reporting Information [Line Items]
Net premiums earned	6,353	6,577
Casualty Segment | Other casualty
Segment Reporting Information [Line Items]
Net premiums earned	18,666	17,539
Property Segment
Segment Reporting Information [Line Items]
Net premiums earned	88,767	67,440
Property Segment | Commercial property
Segment Reporting Information [Line Items]
Net premiums earned	49,262	33,289
Property Segment | Marine
Segment Reporting Information [Line Items]
Net premiums earned	30,636	26,729
Property Segment | Other property
Segment Reporting Information [Line Items]
Net premiums earned	8,869	7,422
Surety Segment
Segment Reporting Information [Line Items]
Net premiums earned	32,925	29,956
Surety Segment | Commercial
Segment Reporting Information [Line Items]
Net premiums earned	12,418	11,703
Surety Segment | Miscellaneous
Segment Reporting Information [Line Items]
Net premiums earned	12,047	11,353
Surety Segment | Contract
Segment Reporting Information [Line Items]
Net premiums earned	$ 8,460	$ 6,900