Segment Information - Schedule of Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Gross written premiums	$ 375,170	$ 388,228	$ 380,003	$ 327,334	$ 295,310	$ 279,969	$ 293,378	$ 298,116	$ 1,470,735	$ 1,166,773	$ 1,081,905
Net earned premiums									823,746	815,398	741,109
Segment revenues	241,150	$ 231,474	$ 220,374	$ 214,127	214,522	$ 224,692	$ 228,041	$ 218,344	907,125	885,599	817,625
Net investment income									75,652	61,256	61,119
Interest expense									10,596	11,553	8,974
Underwriting profit (loss) of operating segments									17,909	54,909	31,094
Goodwill	181,831				181,831				181,831	181,831	181,831
Segment assets	5,024,405				3,136,776				5,024,405	3,136,776	2,756,695
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Gross written premiums									922,320	656,538	530,120
Net earned premiums									625,528	555,684	463,521
Segment revenues									657,501	580,785	493,853
Net investment income									21,358	14,456	15,014
Interest expense									0	0	0
Underwriting profit (loss) of operating segments									19,157	42,834	29,693
Goodwill	181,831				181,831				181,831	181,831	181,831
Segment assets	2,481,934				972,111				2,481,934	972,111	843,486
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Gross written premiums									387,642	374,346	316,430
Net earned premiums									54,338	55,146	68,110
Segment revenues									61,241	56,717	70,366
Net investment income									3,802	3,262	2,532
Interest expense									0	0	0
Underwriting profit (loss) of operating segments									5,913	6,972	3,166
Goodwill	0				0				0	0	0
Segment assets	786,433				633,689				786,433	633,689	439,416
Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums									160,773	135,889	235,355
Net earned premiums									143,880	204,568	209,478
Segment revenues									183,700	243,178	240,751
Net investment income									46,325	38,838	31,507
Interest expense									0	0	0
Underwriting profit (loss) of operating segments									(7,161)	5,103	(1,765)
Goodwill	0				0				0	0	0
Segment assets	1,699,473				1,453,754				1,699,473	1,453,754	1,379,866
Corporate and other segment
Segment Reporting Information [Line Items]
Gross written premiums									0	0	0
Net earned premiums									0	0	0
Segment revenues									4,683	4,919	12,655
Net investment income									4,167	4,700	12,066
Interest expense									10,596	11,553	8,974
Underwriting profit (loss) of operating segments									0	0	0
Goodwill	0				0				0	0	0
Segment assets	$ 56,565				$ 77,222				$ 56,565	$ 77,222	$ 93,927