Share-based compensation - Share options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Additional Information
Total share-based compensation expense	$ 1,319		$ 905		$ 3,440	$ 1,181
Ordinary Shares
Number of Shares
Exercised	(4,000)	(1,071)	(14,301)	(3)
Stock option
Number of Shares
Outstanding number of shares at beginning		2,634,346			2,634,346
Granted					1,174,434
Exercised					(5,071)
Forfeited					(149,697)
Outstanding number of shares at end	3,654,012				3,654,012		2,634,346
Number of shares, vested and expected to vest	3,654,012				3,654,012
Number of shares, options exercisable	1,168,098				1,168,098
Weighted Average Exercise Price
Weighted average exercise price at beginning		$ 9.57			$ 9.57
Granted					10.88
Exercised					2.29
Forfeited					10.72
Weighted average exercise price at ending	$ 9.91				9.91		$ 9.57
Weighted average exercise price, vested and expected to vest	9.91				9.91
Weighted average exercise price, options exercisable	$ 8.06				$ 8.06
Weighted Average Contractual Term
Weighted average contractual term, outstanding					8 years 11 months 9 days		9 years 15 days
Weighted average contractual term, vested and expected to vest					8 years 11 months 9 days
Weighted average contractual term ,options exercisable					8 years 2 months 5 days
Aggregate Intrinsic Value
Aggregate intrinsic value outstanding	$ 21,378				$ 21,378		$ 2,868
Granted					$ 7.08	$ 8.26
Exercised	100		$ 100		$ 100	$ 100
Aggregate intrinsic value, vested and expected to vest	21,378				21,378
Aggregate intrinsic value, options exercisable	9,655				$ 9,655
Additional Information
Granted					$ 7.08	$ 8.26
Total share-based compensation expense	1,300		600		$ 3,400	$ 800
Exercised	$ 100		$ 100		$ 100	$ 100