Nature of the business and basis of presentation- Liquidity (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Nature of business and basis of presentation
Net loss	$ 10,217	$ 6,503	$ 4,979	$ 2,534	$ 16,720	$ 7,513
Accumulated deficit	$ 86,662				$ 86,662		$ 69,942