Net loss per share - Basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net loss attributable to ordinary shareholders	$ (10,138)	$ (9,482)	$ (33,577)	$ (26,202)
Denominator:
Weighted average ordinary shares outstanding, basic and diluted	19,426,833	17,900,978	18,504,013	8,734,943
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (0.52)	$ (0.53)	$ (1.81)	$ (3.00)