Segment Information - Summary of Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Gross written premiums	$ 293,378	$ 281,475	$ 591,494	$ 505,654
Net earned premiums	208,210	184,077	409,152	338,764
Underwriting profit of insurance segments	12,834	10,259	27,500	20,994
Net investment income	16,135	13,714	29,391	30,447
Interest expense	2,946	2,224	5,468	4,347
Segment revenues	228,041	202,394	446,385	378,796
Segment goodwill	181,831	181,831	181,831	181,831	$ 181,831
Segment assets	2,937,376	2,532,053	2,937,376	2,532,053	$ 2,756,695
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums	165,398	138,004	332,884	246,999
Net earned premiums	139,127	117,268	269,098	211,117
Underwriting profit of insurance segments	10,117	11,729	21,416	20,529
Net investment income	4,350	3,841	7,392	7,024
Interest expense	0	0	0	0
Segment revenues	147,012	125,484	284,339	226,574
Segment goodwill	181,831	181,831	181,831	181,831
Segment assets	907,314	779,428	907,314	779,428
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums	97,100	76,771	184,501	149,235
Net earned premiums	14,266	17,760	27,606	34,013
Underwriting profit of insurance segments	988	553	2,611	1,395
Net investment income	839	631	1,550	1,267
Interest expense	0	0	0	0
Segment revenues	15,436	18,435	29,391	35,359
Segment goodwill	0	0	0	0
Segment assets	566,196	369,515	566,196	369,515
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	30,880	66,700	74,109	109,420
Net earned premiums	54,817	49,049	112,448	93,634
Underwriting profit of insurance segments	1,729	(2,023)	3,473	(930)
Net investment income	9,662	7,386	17,679	14,510
Interest expense	0	0	0	0
Segment revenues	64,254	56,574	129,780	108,720
Segment goodwill	0	0	0	0
Segment assets	1,378,305	1,268,560	1,378,305	1,268,560
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	1,284	1,856	2,770	7,646
Interest expense	2,946	2,224	5,468	4,347
Segment revenues	1,339	1,901	2,875	8,143
Segment goodwill	0	0	0	0
Segment assets	$ 85,561	$ 114,550	$ 85,561	$ 114,550