Net loss per share - Basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss attributable to ordinary shareholders									$ (51,010)	$ (30,608)	$ (21,846)
Denominator:
Weighted average ordinary shares outstanding, basic and diluted	21,057,855	19,426,833	18,077,770	17,997,929	17,926,165	17,900,978	7,298,139	834,043	19,145,938	11,045,370	438,862
Net loss per share attributable to ordinary shareholders, basic and diluted	$ (0.83)	$ (0.52)	$ (0.67)	$ (0.63)	$ (0.25)	$ (0.53)	$ (1.40)	$ (7.80)	$ (2.66)	$ (2.77)	$ (49.78)