Net loss per share (Details) - USD ($) $ / shares in Units, $ 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
Numerator:
Net loss	$ (15,554)	$ (7,228)	$ (5,362)	$ (11,148)	$ (8,673)	$ (421)	$ (28,144)	$ (20,242)
Accretion of redeemable convertible preferred stock to redemption value				(251)				(321)
Net loss attributable to common stockholders, basic and diluted	$ (15,554)			$ (11,399)			$ (28,144)	$ (20,563)
Denominator:
Weighted average common shares outstanding, basic and diluted	28,024,779			127,141			10,589,119	86,884
Net loss per share attributable to common stockholders, basic and diluted	$ (0.56)			$ (89.66)			$ (2.66)	$ (236.67)
Number of weighted average restricted common stock	42,667,000			99,259,000			56,679,000	139,516,000