Underwriting information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Rate	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Revenue from External Customer [Line Items]
Gross written premiums	$ 72,164	$ 69,546	$ 69,981	$ 63,847	$ 56,943	$ 55,633	$ 57,753	$ 52,862	$ 275,538	$ 223,191	$ 188,478
Number of operating segments									1
Construction [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 50,879	48,587	42,234
Small Business [Member]
Revenue from External Customer [Line Items]
Gross written premiums									44,368	34,896	27,333
Energy [Member]
Revenue from External Customer [Line Items]
Gross written premiums									25,815	22,898	16,157
Excess Casualty [Member]
Revenue from External Customer [Line Items]
Gross written premiums									26,450	20,260	17,799
General Casualty [Member]
Revenue from External Customer [Line Items]
Gross written premiums									17,625	15,865	16,162
Products Liability [Member]
Revenue from External Customer [Line Items]
Gross written premiums									20,049	14,288	10,140
Professional Liability [Member]
Revenue from External Customer [Line Items]
Gross written premiums									16,717	12,956	14,212
Life Sciences [Member]
Revenue from External Customer [Line Items]
Gross written premiums									14,505	12,408	10,897
Allied Health [Member]
Revenue from External Customer [Line Items]
Gross written premiums									16,815	10,645	9,344
Health Care [Member]
Revenue from External Customer [Line Items]
Gross written premiums									5,725	6,235	6,594
Commercial Property [Member]
Revenue from External Customer [Line Items]
Gross written premiums									9,166	5,609	4,835
Management Liability [Member]
Revenue from External Customer [Line Items]
Gross written premiums									8,161	4,123	2,244
Environmental [Member]
Revenue from External Customer [Line Items]
Gross written premiums									2,924	2,692	1,931
Public Entity [Member]
Revenue from External Customer [Line Items]
Gross written premiums									1,193	1,265	875
Inland Marine [Member]
Revenue from External Customer [Line Items]
Gross written premiums									2,046	967	910
Commercial Insurance [Member]
Revenue from External Customer [Line Items]
Gross written premiums									1,096	816	459
Commercial [Member]
Revenue from External Customer [Line Items]
Gross written premiums									263,534	214,510	182,126
Personal Insurance [Member]
Revenue from External Customer [Line Items]
Gross written premiums									12,004	8,681	6,352
Personal [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 12,004	$ 8,681	$ 6,352
Sales Revenue, Services, Net [Member]
Revenue from External Customer [Line Items]
Number of insurance brokers									2
Major Insurance Brokers, Group One [Member] | Sales Revenue, Services, Net [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 32,700
Concentration Risk, Percentage | Rate									11.90%
Major Insurance Brokers, Group Two [Member] | Sales Revenue, Services, Net [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 29,400
Concentration Risk, Percentage | Rate									10.70%