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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Share-based compensation - Share options (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Additional Information
Total share-based compensation expense		$ 6,514,000	$ 3,083,000	$ 1,023,000
Ordinary Shares
Number of Shares
Exercised		(79,158)	(85,839)	(9,361)
Stock option
Number of Shares
Outstanding number of shares at beginning		2,634,346
Granted		1,370,984
Exercised		(79,158)
Forfeited		(189,509)
Outstanding number of shares at end	3,654,012	3,736,663	2,634,346
Number of shares, vested and expected to vest		3,736,663
Number of shares, options exercisable		1,518,721
Weighted Average Exercise Price
Weighted average exercise price at beginning		$ 9.57
Granted		12.00
Exercised		3.42
Forfeited		11.10
Weighted average exercise price at ending		10.51	$ 9.57
Weighted average exercise price, vested and expected to vest		10.51
Weighted average exercise price, options exercisable		$ 8.81
Weighted Average Contractual Term
Weighted average contractual term, outstanding		8 years 6 months 15 days	9 years 15 days
Weighted average contractual term, vested and expected to vest		8 years 6 months 15 days
Weighted average contractual term ,options exercisable		7 years 10 months 24 days
Aggregate Intrinsic Value
Aggregate intrinsic value outstanding		$ 27,553,000	$ 6,107,000
Granted		$ 7.87	$ 6.07	$ 3.73
Exercised		$ 1,300,000	$ 600,000	$ 23,000
Aggregate intrinsic value, vested and expected to vest		27,553,000
Aggregate intrinsic value, options exercisable		$ 13,729,000
Additional Information
Granted		$ 7.87	$ 6.07	$ 3.73
Total share-based compensation expense		$ 6,500,000	$ 2,700,000	$ 800,000
Exercised		1,300,000	600,000	$ 23,000
Performance based option
Number of Shares
Granted				70,875
Additional Information
Total share-based compensation expense		$ 100,000	$ 100,000	$ 700,000
Performance based option \| Ordinary Shares \| First anniversary
Additional Information
Vesting Percentage				20.00%
Performance based option \| Ordinary Shares \| 36 equal monthly installments
Additional Information
Vesting Percentage			60.00%
Number of equal monthly installments for vesting remaining awards				36 months
Performance based option \| Ordinary Shares \| Earlier of the fourth anniversary
Additional Information
Vesting Percentage				20.00%