Segment Information - Summary of Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross written premiums	$ 338,351	$ 260,166	$ 844,005	$ 563,908
Net earned premiums	202,116	133,149	540,880	368,834
Underwriting profit (loss) of insurance segments	7,864	13,243	28,858	33,370
Net investment income	14,880	15,797	45,327	38,622
Interest expense	2,304	2,079	6,651	6,294
Segment revenues	220,866	151,365	599,662	417,205
Segment goodwill	181,831	181,831	181,831	181,831	$ 181,831
Segment assets	2,721,652	2,347,911	2,721,652	2,347,911	$ 2,346,533
Reportable segments | Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums	140,425	99,882	387,424	279,417
Net earned premiums	123,606	81,672	334,723	217,742
Underwriting profit (loss) of insurance segments	9,946	12,408	30,475	31,395
Net investment income	3,727	5,236	10,751	13,601
Interest expense	0	0	0	0
Segment revenues	131,033	88,671	357,607	239,401
Segment goodwill	181,831	181,831	181,831	181,831
Segment assets	821,668	728,231	821,668	728,231
Reportable segments | Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums	84,838	56,119	234,073	119,007
Net earned premiums	19,324	13,204	53,337	36,816
Underwriting profit (loss) of insurance segments	851	702	2,246	1,302
Net investment income	626	647	1,893	1,898
Interest expense	0	0	0	0
Segment revenues	19,992	13,890	55,351	39,266
Segment goodwill	0	0	0	0
Segment assets	405,209	261,063	405,209	261,063
Reportable segments | Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	113,088	104,165	222,508	165,484
Net earned premiums	59,186	38,273	152,820	114,276
Underwriting profit (loss) of insurance segments	(2,933)	133	(3,863)	673
Net investment income	8,590	7,085	23,100	20,248
Interest expense	0	0	0	0
Segment revenues	67,860	44,808	176,580	134,420
Segment goodwill	0	0	0	0
Segment assets	1,372,234	1,258,522	1,372,234	1,258,522
Corporate and Other | 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 insurance segments	0	0	0	0
Net investment income	1,937	2,829	9,583	2,875
Interest expense	2,304	2,079	6,651	6,294
Segment revenues	1,981	3,996	10,124	4,118
Segment goodwill	0	0	0	0
Segment assets	$ 122,541	$ 100,095	$ 122,541	$ 100,095