Segment Information - Gross Written Premiums (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]
Total 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
Excess and Surplus Lines
Segment Reporting Information [Line Items]
Total gross written premiums									922,320	656,538	530,120
Excess and Surplus Lines | Commercial Auto
Segment Reporting Information [Line Items]
Total gross written premiums									405,565	322,126	247,960
Excess and Surplus Lines | Excess Casualty
Segment Reporting Information [Line Items]
Total gross written premiums									118,954	66,452	51,160
Excess and Surplus Lines | General Casualty
Segment Reporting Information [Line Items]
Total gross written premiums									115,832	54,127	38,097
Excess and Surplus Lines | Manufacturers and Contractors
Segment Reporting Information [Line Items]
Total gross written premiums									105,096	79,160	85,719
Excess and Surplus Lines | Energy
Segment Reporting Information [Line Items]
Total gross written premiums									45,442	33,942	29,704
Excess and Surplus Lines | Excess Property
Segment Reporting Information [Line Items]
Total gross written premiums									31,606	16,963	14,447
Excess and Surplus Lines | Allied Health
Segment Reporting Information [Line Items]
Total gross written premiums									26,713	30,450	19,181
Excess and Surplus Lines | Life Sciences
Segment Reporting Information [Line Items]
Total gross written premiums									24,462	16,636	12,981
Excess and Surplus Lines | Small Business
Segment Reporting Information [Line Items]
Total gross written premiums									19,725	14,808	11,307
Excess and Surplus Lines | Environmental
Segment Reporting Information [Line Items]
Total gross written premiums									16,539	10,499	7,920
Excess and Surplus Lines | Professional Liability
Segment Reporting Information [Line Items]
Total gross written premiums									6,441	5,916	6,326
Excess and Surplus Lines | Sports and Entertainment
Segment Reporting Information [Line Items]
Total gross written premiums									4,212	3,685	3,021
Excess and Surplus Lines | Medical Professionals
Segment Reporting Information [Line Items]
Total gross written premiums									1,733	1,774	2,297
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Total gross written premiums									387,642	374,346	316,430
Casualty Reinsurance
Segment Reporting Information [Line Items]
Total gross written premiums									$ 160,773	$ 135,889	$ 235,355