Net loss per share - Basic EPS and Diluted EPS (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic and Diluted EPS:
Net loss	$ (20,603)	$ (31,488)	$ (16,628)	$ (16,657)	$ (52,091)	$ (33,285)
Denominator
Weighted average number of shares outstanding	14,981,756		12,355,969		14,556,986	12,178,376
Basic and Diluted EPS	$ (1.38)		$ (1.35)		$ (3.58)	$ (2.73)