Underwriting information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Rate	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Revenue from External Customer [Line Items]
Gross written premiums	$ 56,943	$ 55,633	$ 57,753	$ 52,862	$ 47,466	$ 47,823	$ 50,107	$ 43,082	$ 223,191	$ 188,478	$ 177,009
Number of operating segments									1
Construction [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 48,587	42,234	36,932
Small Business [Member]
Revenue from External Customer [Line Items]
Gross written premiums									34,896	27,333	21,468
Energy [Member]
Revenue from External Customer [Line Items]
Gross written premiums									22,898	16,157	19,022
Excess Casualty [Member]
Revenue from External Customer [Line Items]
Gross written premiums									20,260	17,799	16,194
General Casualty [Member]
Revenue from External Customer [Line Items]
Gross written premiums									15,865	16,162	20,511
Products Liability [Member]
Revenue from External Customer [Line Items]
Gross written premiums									14,288	10,140	9,480
Professional Liability [Member]
Revenue from External Customer [Line Items]
Gross written premiums									12,956	14,212	14,636
Life Sciences [Member]
Revenue from External Customer [Line Items]
Gross written premiums									12,408	10,897	11,935
Allied Health [Member]
Revenue from External Customer [Line Items]
Gross written premiums									10,645	9,344	8,644
Healthcare [Member]
Revenue from External Customer [Line Items]
Gross written premiums									6,235	6,594	6,579
Commercial Property [Member]
Revenue from External Customer [Line Items]
Gross written premiums									5,609	4,835	6,181
Management Liability [Member]
Revenue from External Customer [Line Items]
Gross written premiums									4,123	2,244	420
Environmental [Member]
Revenue from External Customer [Line Items]
Gross written premiums									2,692	1,931	1,005
Public Entity [Member]
Revenue from External Customer [Line Items]
Gross written premiums									1,265	875	0
Inland Marine [Member]
Revenue from External Customer [Line Items]
Gross written premiums									967	910	195
Commercial Insurance [Member]
Revenue from External Customer [Line Items]
Gross written premiums									816	459	0
Commercial [Member]
Revenue from External Customer [Line Items]
Gross written premiums									214,510	182,126	173,202
Personal Insurance [Member]
Revenue from External Customer [Line Items]
Gross written premiums									8,681	6,352	3,807
Personal [Member]
Revenue from External Customer [Line Items]
Gross written premiums									8,681	$ 6,352	$ 3,807
Major Insurance Brokers, Group One [Member] | Sales Revenue, Services, Net [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 26,400
Concentration risk, percentage | Rate									11.80%
Major Insurance Brokers, Group Two [Member] | Sales Revenue, Services, Net [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 24,400
Concentration risk, percentage | Rate									10.90%
Major Insurance Brokers, Group Three [Member] [Member] | Sales Revenue, Services, Net [Member]
Revenue from External Customer [Line Items]
Gross written premiums									$ 23,100
Concentration risk, percentage | Rate									10.40%