Segment Information - Summary of Segment Results (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Gross written premiums	$ 298,116	$ 224,179
Net earned premiums	200,942	154,687
Underwriting profit (loss) of insurance segments	14,666	10,735
Net investment income	13,256	16,733
Interest expense	2,522	2,123
Segment revenues	218,344	176,402
Segment goodwill	181,831	181,831	$ 181,831
Segment assets	2,862,010	2,411,325	$ 2,756,695
Reportable segments | Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums	167,486	108,995
Net earned premiums	129,971	93,849
Underwriting profit (loss) of insurance segments	11,299	8,800
Net investment income	3,042	3,183
Interest expense	0	0
Segment revenues	137,327	101,090
Segment goodwill	181,831	181,831
Segment assets	909,963	749,617
Reportable segments | Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums	87,401	72,464
Net earned premiums	13,340	16,253
Underwriting profit (loss) of insurance segments	1,623	842
Net investment income	711	636
Interest expense	0	0
Segment revenues	13,955	16,924
Segment goodwill	0	0
Segment assets	495,096	335,806
Reportable segments | Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums	43,229	42,720
Net earned premiums	57,631	44,585
Underwriting profit (loss) of insurance segments	1,744	1,093
Net investment income	8,017	7,124
Interest expense	0	0
Segment revenues	65,526	52,146
Segment goodwill	0	0
Segment assets	1,368,872	1,211,896
Corporate and Other | Corporate and other
Segment Reporting Information [Line Items]
Gross written premiums	0	0
Net earned premiums	0	0
Underwriting profit (loss) of insurance segments	0	0
Net investment income	1,486	5,790
Interest expense	2,522	2,123
Segment revenues	1,536	6,242
Segment goodwill	0	0
Segment assets	$ 88,079	$ 114,006