Segment Information - Narrative (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) company insured agency broker	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) company insured Underwriting_division agency broker Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment									4
Fee income									$ 9,100	$ 13,900	$ 17,000
Number of underwriting divisions | Underwriting_division									15
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]
Number of brokers | broker	3								3
Number of underwriting divisions | Underwriting_division									13
Gross written premiums									$ 922,320	656,538	530,120
Number of insured | insured	1								1
Excess and Surplus Lines | Broker one
Segment Reporting Information [Line Items]
Gross written premiums									$ 472,600
Consolidated gross written premiums percentage									32.10%
Gross written premiums percentage									51.20%
Excess and Surplus Lines | Broker two
Segment Reporting Information [Line Items]
Gross written premiums									$ 115,200
Consolidated gross written premiums percentage									7.80%
Gross written premiums percentage									12.50%
Excess and Surplus Lines | Broker three
Segment Reporting Information [Line Items]
Gross written premiums									$ 98,600
Consolidated gross written premiums percentage									6.70%
Gross written premiums percentage									10.70%
Excess and Surplus Lines | Insured one
Segment Reporting Information [Line Items]
Fee income									$ 9,100
Gross written premiums									$ 374,200
Consolidated gross written premiums percentage									25.40%
Gross written premiums percentage									40.60%
Specialty Admitted Insurance
Segment Reporting Information [Line Items]
Number of underwriting divisions | Underwriting_division									1
Gross written premiums									$ 387,642	374,346	316,430
Number of agencies | agency	2								2
Specialty Admitted Insurance | Agency one
Segment Reporting Information [Line Items]
Gross written premiums									$ 146,500
Consolidated gross written premiums percentage									10.00%
Gross written premiums percentage									37.80%
Specialty Admitted Insurance | Agency two
Segment Reporting Information [Line Items]
Gross written premiums									$ 53,200
Consolidated gross written premiums percentage									3.60%
Gross written premiums percentage									13.70%
Casualty Reinsurance
Segment Reporting Information [Line Items]
Number of brokers | broker	4								4
Number of underwriting divisions | Underwriting_division									1
Gross written premiums									$ 160,773	$ 135,889	$ 235,355
Number of unaffiliated ceding companies | company	3								3
Casualty Reinsurance | Broker one
Segment Reporting Information [Line Items]
Gross written premiums									$ 53,700
Consolidated gross written premiums percentage									3.70%
Gross written premiums percentage									33.40%
Casualty Reinsurance | Broker two
Segment Reporting Information [Line Items]
Gross written premiums									$ 45,100
Consolidated gross written premiums percentage									3.10%
Gross written premiums percentage									28.10%
Casualty Reinsurance | Broker three
Segment Reporting Information [Line Items]
Gross written premiums									$ 42,400
Consolidated gross written premiums percentage									2.90%
Gross written premiums percentage									26.40%
Casualty Reinsurance | Broker four
Segment Reporting Information [Line Items]
Gross written premiums									$ 16,700
Consolidated gross written premiums percentage									1.10%
Gross written premiums percentage									10.40%
Casualty Reinsurance | Unaffiliated ceding company one
Segment Reporting Information [Line Items]
Gross written premiums									$ 96,400
Consolidated gross written premiums percentage									6.60%
Gross written premiums percentage									59.90%
Casualty Reinsurance | Unaffiliated ceding company two
Segment Reporting Information [Line Items]
Gross written premiums									$ 30,700
Consolidated gross written premiums percentage									2.10%
Gross written premiums percentage									19.10%
Casualty Reinsurance | Unaffiliated ceding company three
Segment Reporting Information [Line Items]
Gross written premiums									$ 22,600
Consolidated gross written premiums percentage									1.50%
Gross written premiums percentage									14.00%