Nature of the business (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ 199,327		$ 199,327		$ 173,388
Net loss	$ 14,383	$ 7,742	$ 25,939	$ 22,639	$ 30,704