Net loss per share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (43,755)	$ (81,714)
Accretion of redeemable convertible preferred stock to redemption value		(520)
Net loss attributable to common stockholders, basic and diluted	$ (43,755)	$ (82,234)
Denominator:
Weighted average common shares outstanding, basic and diluted	15,070,728	103,403
Net loss per share attributable to common stockholders, basic and diluted	$ (2.90)	$ (795.28)
Number of weighted average restricted common stock	125,790	49,568,000