Nature of the business (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
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ 229,408				$ 229,408		$ 207,885
Net loss	$ 10,007	$ 11,341	$ 7,801	$ 3,755	$ 21,348	$ 11,556