Underwriting information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	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 ($) Rate	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Revenue from External Customer [Line Items]
Gross written premiums	$ 112,137	$ 97,984	$ 94,947	$ 84,626	$ 72,164	$ 69,546	$ 69,981	$ 63,847	$ 389,694	$ 275,538	$ 223,191
Number of operating segments									1
Construction [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 71,035	50,879	48,587
Small Business [Member]
Revenue from External Customer [Line Items]
Gross written premiums									63,181	44,368	34,896
Energy [Member]
Revenue from External Customer [Line Items]
Gross written premiums									15,371	15,586	13,544
Excess Casualty [Member]
Revenue from External Customer [Line Items]
Gross written premiums									51,225	37,398	30,260
General Casualty [Member]
Revenue from External Customer [Line Items]
Gross written premiums									23,279	17,625	15,865
Products Liability [Member]
Revenue from External Customer [Line Items]
Gross written premiums									26,333	20,049	14,288
Professional Liability [Member]
Revenue from External Customer [Line Items]
Gross written premiums									20,029	16,717	12,956
Life Sciences [Member]
Revenue from External Customer [Line Items]
Gross written premiums									17,821	14,505	12,408
Allied Health [Member]
Revenue from External Customer [Line Items]
Gross written premiums									23,962	16,815	10,645
Health Care [Member]
Revenue from External Customer [Line Items]
Gross written premiums									5,963	5,725	6,235
Commercial Property [Member]
Revenue from External Customer [Line Items]
Gross written premiums									29,115	9,166	5,609
Management Liability [Member]
Revenue from External Customer [Line Items]
Gross written premiums									14,820	8,161	4,123
Environmental [Member]
Revenue from External Customer [Line Items]
Gross written premiums									5,179	2,205	2,046
Public Entity [Member]
Revenue from External Customer [Line Items]
Gross written premiums									580	1,193	1,265
Inland Marine [Member]
Revenue from External Customer [Line Items]
Gross written premiums									3,467	2,046	967
Commercial Insurance [Member]
Revenue from External Customer [Line Items]
Gross written premiums									1,674	1,096	816
Commercial [Member]
Revenue from External Customer [Line Items]
Gross written premiums									373,034	263,534	214,510
Personal Insurance [Member]
Revenue from External Customer [Line Items]
Gross written premiums									16,660	12,004	8,681
Personal [Member]
Revenue from External Customer [Line Items]
Gross written premiums									16,660	$ 12,004	$ 8,681
Revenue Benchmark [Member]
Revenue from External Customer [Line Items]
Number of insurance brokers										2
Major Insurance Brokers, Group One [Member] | Revenue Benchmark [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 47,200
Concentration Risk, Percentage | Rate									12.10%
Major Insurance Brokers, Group Two [Member] | Revenue Benchmark [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 41,100
Concentration Risk, Percentage | Rate									10.50%