Share-based compensation - Share options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Additional Information
Total share-based compensation expense	$ 1,510			$ 819			$ 4,950	$ 2,000
Ordinary Shares
Number of Shares
Exercised	(13,483)	(4,000)	(1,071)	(2,138)	(14,301)	(3)
Stock option
Number of Shares
Outstanding number of shares at beginning	3,654,012		2,634,346				2,634,346
Granted							1,319,484
Exercised							(18,554)
Forfeited							(155,491)
Outstanding number of shares at end	3,779,785	3,654,012					3,779,785		2,634,346
Number of shares, vested and expected to vest	3,779,785						3,779,785
Number of shares, options exercisable	1,377,257						1,377,257
Weighted Average Exercise Price
Weighted average exercise price at beginning			$ 9.57				$ 9.57
Granted							11.69
Exercised							5.33
Forfeited							10.69
Weighted average exercise price at ending	$ 10.24						10.24		$ 9.57
Weighted average exercise price, vested and expected to vest	10.24						10.24
Weighted average exercise price, options exercisable	$ 8.39						$ 8.39
Weighted Average Contractual Term
Weighted average contractual term, outstanding							8 years 9 months 4 days		9 years 15 days
Weighted average contractual term, vested and expected to vest							8 years 9 months 4 days
Weighted average contractual term ,options exercisable							8 years 29 days
Aggregate Intrinsic Value
Aggregate intrinsic value outstanding	$ 33,340						$ 33,340		$ 6,107
Granted							$ 7.57	$ 4.81
Exercised	200			$ 100			$ 200	$ 100
Aggregate intrinsic value, vested and expected to vest	33,340						33,340
Aggregate intrinsic value, options exercisable	15,580						$ 15,580
Additional Information
Granted							$ 7.57	$ 4.81
Total share-based compensation expense	1,500			800			$ 5,000	$ 1,600
Exercised	$ 200			$ 100			$ 200	$ 100