Nature of the business (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ 184,944		$ 184,944		$ 173,388
Net loss	$ 7,801	$ 7,380	$ 11,556	$ 14,897	$ 30,704