Share-based compensation - Share options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Number of Shares
Outstanding number of shares at beginning		3,736,663			3,736,663
Granted					1,424,999
Exercised					(189,211)
Forfeited					(51,620)
Outstanding number of shares at end	4,920,831				4,920,831		3,736,663
Number of shares, vested and expected to vest	4,920,831				4,920,831
Number of shares, options exercisable	2,169,250				2,169,250
Weighted Average Exercise Price
Weighted average exercise price at beginning		$ 10.51			$ 10.51
Granted					19.72
Exercised					9.83
Forfeited					15.38
Weighted average exercise price at ending	$ 13.16				13.16		$ 10.51
Weighted average exercise price, vested and expected to vest	13.16				13.16
Weighted average exercise price, options exercisable	$ 9.90				$ 9.90
Weighted Average Contractual Term
Weighted average contractual term, outstanding					8 years 8 months 8 days		8 years 6 months 14 days
Weighted average contractual term, vested and expected to vest					8 years 8 months 8 days
Weighted average contractual term ,options exercisable					8 years 2 months 4 days
Aggregate Intrinsic Value
Aggregate intrinsic value outstanding	$ 80,172				$ 80,172		$ 27,553
Aggregate intrinsic value, vested and expected to vest	80,172				80,172
Aggregate intrinsic value, options exercisable	39,782				39,782
Additional Information
Total share-based compensation expense	$ 2,575		$ 1,319		$ 6,396	$ 3,440
Ordinary Shares
Number of Shares
Exercised	(125,666)	(63,545)	(4,000)	(1,071)
Stock option
Number of Shares
Outstanding number of shares at end			3,654,012			3,654,012
Aggregate Intrinsic Value
Granted					$ 13.30	$ 7.08
Exercised	$ 2,200		$ 0		$ 3,700	$ 100
Additional Information
Granted					$ 13.30	$ 7.08
Total share-based compensation expense	2,600		1,300		$ 6,400	$ 3,400
Exercised	$ 2,200		$ 0		$ 3,700	$ 100