Segment Information - Summary of Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Gross written premiums	$ 301,639	$ 380,003	$ 585,480	$ 707,337
Net earned premiums	148,815	199,114	294,733	389,266
Underwriting profit of insurance segments	14,888	17,008	22,219	32,060
Net investment income	15,350	17,535	36,186	36,966
Interest expense	2,965	2,684	5,841	5,492
Total revenues	186,749	220,374	297,033	434,501
Segment goodwill	181,831	181,831	181,831	181,831	$ 181,831
Segment assets	5,009,201	3,451,107	5,009,201	3,451,107	$ 5,024,405
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums	186,994	260,277	323,191	446,826
Net earned premiums	100,849	150,921	200,588	292,593
Underwriting profit of insurance segments	16,095	15,810	24,207	28,912
Net investment income	3,336	4,229	11,277	9,773
Interest expense	0	0	0	0
Total revenues	113,343	158,574	197,172	311,011
Segment goodwill	181,831	181,831	181,831	181,831
Segment assets	2,304,169	1,122,080	2,304,169	1,122,080
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums	88,440	89,472	191,242	192,425
Net earned premiums	14,392	13,086	27,675	25,446
Underwriting profit of insurance segments	1,430	1,298	442	2,921
Net investment income	812	918	1,740	1,815
Interest expense	0	0	0	0
Total revenues	15,870	14,940	28,543	28,676
Segment goodwill	0	0	0	0
Segment assets	838,753	721,506	838,753	721,506
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	26,205	30,254	71,047	68,086
Net earned premiums	33,574	35,107	66,470	71,227
Underwriting profit of insurance segments	(2,637)	(100)	(2,430)	227
Net investment income	10,534	11,986	22,138	23,158
Interest expense	0	0	0	0
Total revenues	56,794	46,318	70,074	92,328
Segment goodwill	0	0	0	0
Segment assets	1,808,921	1,544,565	1,808,921	1,544,565
Corporate and Other
Segment Reporting Information [Line Items]
Gross written premiums	0	0	0	0
Net earned premiums	0	0	0	0
Underwriting profit of insurance segments	0	0	0	0
Net investment income	668	402	1,031	2,220
Interest expense	2,965	2,684	5,841	5,492
Total revenues	742	542	1,244	2,486
Segment goodwill	0	0	0	0
Segment assets	$ 57,358	$ 62,956	$ 57,358	$ 62,956