Nature of the business (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Accumulated deficit	$ 238,279						$ 238,279		$ 207,885
Net loss	$ 8,871	$ 10,007	$ 11,341	$ 14,383	$ 7,801	$ 3,755	$ 30,219	$ 25,939