Segment Information - Summary of Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross written premiums	$ 281,475	$ 170,671	$ 505,654	$ 303,742
Net earned premiums	184,077	118,555	338,764	235,685
Underwriting profit (loss) of insurance segments	10,259	10,114	20,994	20,127
Net investment income	13,714	11,553	30,447	22,825
Interest expense	2,224	2,041	4,347	4,215
Segment revenues	202,394	134,511	378,796	265,840
Segment goodwill	181,831	181,831	181,831	181,831	$ 181,831
Segment assets	2,532,053	2,189,861	2,532,053	2,189,861	$ 2,346,533
Reportable segments | Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums	138,004	97,427	246,999	179,535
Net earned premiums	117,268	70,565	211,117	136,070
Underwriting profit (loss) of insurance segments	11,729	9,783	20,529	18,987
Net investment income	3,841	5,079	7,024	8,365
Interest expense	0	0	0	0
Segment revenues	125,484	78,940	226,574	150,730
Segment goodwill	181,831	181,831	181,831	181,831
Segment assets	779,428	731,324	779,428	731,324
Reportable segments | Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums	76,771	34,201	149,235	62,888
Net earned premiums	17,760	12,207	34,013	23,612
Underwriting profit (loss) of insurance segments	553	125	1,395	600
Net investment income	631	640	1,267	1,251
Interest expense	0	0	0	0
Segment revenues	18,435	13,329	35,359	25,376
Segment goodwill	0	0	0	0
Segment assets	369,515	215,034	369,515	215,034
Reportable segments | Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	66,700	39,043	109,420	61,319
Net earned premiums	49,049	35,783	93,634	76,003
Underwriting profit (loss) of insurance segments	(2,023)	206	(930)	540
Net investment income	7,386	6,936	14,510	13,163
Interest expense	0	0	0	0
Segment revenues	56,574	43,304	108,720	89,612
Segment goodwill	0	0	0	0
Segment assets	1,268,560	1,148,489	1,268,560	1,148,489
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	1,856	(1,102)	7,646	46
Interest expense	2,224	2,041	4,347	4,215
Segment revenues	1,901	(1,062)	8,143	122
Segment goodwill	0	0	0	0
Segment assets	$ 114,550	$ 95,014	$ 114,550	$ 95,014