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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 ($)	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Additional Information
Total share-based compensation expense	$ 905,000		$ 335,000		$ 1,181,000	$ 587,000
Ordinary Shares
Number of Shares
Exercised	(14,301)	(3)	(359)	(9,002)
Stock option
Number of Shares
Outstanding number of shares at beginning		863,712			863,712
Granted					1,875,820
Exercised					(14,304)
Forfeited					(109,279)
Outstanding number of shares at end	2,615,949				2,615,949		863,712
Number of shares vested and expected to vest	2,615,949				2,615,949
Number of shares options exercisable	326,708				326,708
Weighted Average Exercise Price
Weighted average exercise price at beginning		$ 1.01			$ 1.01
Granted					12.13
Exercised					1.49
Forfeited					1.61
Weighted average exercise price at ending	$ 8.96				8.96		$ 1.01
Weighted average exercise price, vested and expected to vest	8.96				8.96
Weighted average exercise price, options exercisable	$ 3.03				$ 3.03
Weighted Average Contractual Term
Weighted average contractual term outstanding					9 years 4 months 21 days		8 years 9 months
Weighted average contractual term vested and expected to vest					9 years 4 months 21 days
Weighted average contractual term options exercisable					8 years 3 months 7 days
Aggregate Intrinsic Value
Aggregate intrinsic value outstanding	$ 8,313,000				$ 8,313,000		$ 3,292,000
Aggregate intrinsic value, vested and expected to vest	8,313,000				8,313,000
Aggregate intrinsic value, options exercisable	2,441,000				$ 2,441,000
Additional Information
Weighted average grant-date fair value of share options granted (per share)					$ 8.26	$ 1.83
Total share-based compensation expense	600,000		$ 300,000		$ 800,000	$ 500,000
Aggregate intrinsic value of share options	100,000				$ 100,000
Performance based option
Number of Shares
Granted					0	18,719
Additional Information
Total share-based compensation expense	$ 0		$ 300,000		$ 43,000	$ 500,000
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%