Share-based compensation - Share options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Number of Shares
Outstanding number of shares at beginning			3,736,663				3,736,663
Granted							1,509,205
Exercised							(446,600)
Forfeited							(82,102)
Outstanding number of shares at end	4,717,166						4,717,166		3,736,663
Number of shares, vested and expected to vest	4,717,166						4,717,166
Number of shares, options exercisable	2,181,492						2,181,492
Weighted Average Exercise Price
Weighted average exercise price at beginning			$ 10.51				$ 10.51
Granted							20.48
Exercised							10.99
Forfeited							18.08
Weighted average exercise price at ending	$ 13.53						13.53		$ 10.51
Weighted average exercise price, vested and expected to vest	13.53						13.53
Weighted average exercise price, options exercisable	$ 10.00						$ 10.00
Weighted Average Contractual Term
Weighted average contractual term, outstanding							8 years 3 months 14 days		8 years 6 months 14 days
Weighted average contractual term, vested and expected to vest							8 years 3 months 14 days
Weighted average contractual term ,options exercisable							7 years 8 months 23 days
Aggregate Intrinsic Value
Aggregate intrinsic value outstanding	$ 127,324						$ 127,324		$ 27,553
Aggregate intrinsic value, vested and expected to vest	127,324						127,324
Aggregate intrinsic value, options exercisable	68,059						68,059
Additional Information
Total share-based compensation expense	$ 2,688			$ 1,510			$ 9,084	$ 4,950
Ordinary Shares
Number of Shares
Exercised	(257,389)	(125,666)	(63,545)	(13,483)	(4,000)	(1,071)
Stock option
Number of Shares
Outstanding number of shares at beginning				3,654,012
Outstanding number of shares at end					3,654,012
Aggregate Intrinsic Value
Granted							$ 13.83	$ 7.57
Exercised	$ 5,700			$ 200			$ 9,400	$ 200
Additional Information
Granted							$ 13.83	$ 7.57
Total share-based compensation expense	2,700			1,500			$ 9,100	$ 5,000
Exercised	$ 5,700			$ 200			$ 9,400	$ 200