Segment Information - Summary of Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Gross written premiums	$ 311,852	$ 388,228	$ 897,332	$ 1,095,565
Net earned premiums	152,962	213,374	447,695	602,640
Underwriting profit of insurance segments	16,751	(32,802)	38,970	(742)
Net investment income	14,959	17,878	51,145	54,844
Interest expense	2,129	2,594	7,970	8,086
Total revenues	177,465	231,474	474,498	665,975
Segment goodwill	181,831	181,831	181,831	181,831	$ 181,831
Segment assets	4,984,353	3,660,916	4,984,353	3,660,916	$ 5,024,405
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums	179,458	241,045	502,649	687,871
Net earned premiums	104,933	164,759	305,521	457,352
Underwriting profit of insurance segments	15,581	(29,351)	39,788	(439)
Net investment income	3,751	3,467	15,028	13,240
Interest expense	0	0	0	0
Total revenues	112,800	170,734	309,972	481,745
Segment goodwill	181,831	181,831	181,831	181,831
Segment assets	2,243,065	1,186,094	2,243,065	1,186,094
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums	112,589	100,459	303,831	292,884
Net earned premiums	14,985	14,242	42,660	39,688
Underwriting profit of insurance segments	1,859	837	2,301	3,758
Net investment income	807	976	2,547	2,791
Interest expense	0	0	0	0
Total revenues	16,843	16,493	45,386	45,169
Segment goodwill	0	0	0	0
Segment assets	877,619	773,677	877,619	773,677
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	19,805	46,724	90,852	114,810
Net earned premiums	33,044	34,373	99,514	105,600
Underwriting profit of insurance segments	(689)	(4,288)	(3,119)	(4,061)
Net investment income	10,033	11,717	32,171	34,875
Interest expense	0	0	0	0
Total revenues	47,385	42,403	117,459	134,731
Segment goodwill	0	0	0	0
Segment assets	1,806,341	1,631,569	1,806,341	1,631,569
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	368	1,718	1,399	3,938
Interest expense	2,129	2,594	7,970	8,086
Total revenues	437	1,844	1,681	4,330
Segment goodwill	0	0	0	0
Segment assets	$ 57,328	$ 69,576	$ 57,328	$ 69,576