Property and equipment - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 09, 2015 ft²	Sep. 05, 2014 a ft²
Property, Plant and Equipment [Line Items]
Depreciation, Depletion and Amortization | $	$ 2,424	$ 1,803	$ 731
Leaseholds and Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Area of Land					1.5
Building [Member]
Property, Plant and Equipment [Line Items]
Area of Real Estate Property | ft²				7,800	40,000
Land and Building [Member]
Property, Plant and Equipment [Line Items]
Area of Land					2.3