Segment Information - Summary of Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Gross written premiums	$ 399,714	$ 380,146	$ 759,650	$ 753,401
Net earned premiums	186,262	172,705	376,086	333,298
Underwriting profit (loss) of operating segments	25,645	25,734	38,474	(125,571)
Net investment income	14,705	14,348	30,972	29,437
Interest expense	4,049	2,249	6,341	4,465
Segment revenues	184,806	191,567	386,754	374,547
Segment goodwill	181,831	181,831	181,831	181,831	$ 181,831
Segment assets	5,265,275	5,391,816	5,265,275	5,391,816	$ 4,948,550
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums	266,635	214,014	470,917	395,372
Net earned premiums	137,884	117,945	269,185	231,653
Underwriting profit (loss) of operating segments	22,334	26,917	43,791	(124,029)
Net investment income	3,298	3,473	8,840	7,179
Interest expense	0	0	0	0
Segment revenues	131,659	124,018	265,051	242,814
Segment goodwill	181,831	181,831	181,831	181,831
Segment assets	2,096,138	2,211,469	2,096,138	2,211,469
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums	124,967	129,189	250,677	256,225
Net earned premiums	18,141	18,595	37,459	34,952
Underwriting profit (loss) of operating segments	1,252	2,138	1,461	3,404
Net investment income	934	766	1,691	1,588
Interest expense	0	0	0	0
Segment revenues	19,187	21,093	39,550	39,658
Segment goodwill	0	0	0	0
Segment assets	1,135,839	1,034,622	1,135,839	1,034,622
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	8,112	36,943	38,056	101,804
Net earned premiums	30,237	36,165	69,442	66,693
Underwriting profit (loss) of operating segments	2,059	(3,321)	(6,778)	(4,946)
Net investment income	10,441	9,707	20,154	20,263
Interest expense	1,327	0	1,342	0
Segment revenues	33,854	45,987	81,725	91,504
Segment goodwill	0	0	0	0
Segment assets	1,999,687	2,120,625	1,999,687	2,120,625
Corporate and Other
Segment Reporting Information [Line Items]
Gross written premiums	0	0	0	0
Net earned premiums	0	0	0	0
Underwriting profit (loss) of operating segments	0	0	0	0
Net investment income	32	402	287	407
Interest expense	2,722	2,249	4,999	4,465
Segment revenues	106	469	428	571
Segment goodwill	0	0	0	0
Segment assets	$ 33,611	$ 25,100	$ 33,611	$ 25,100