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Number of options outstanding, including both vested and non-vested options.

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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 ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Additional Information
Total share-based compensation expense	$ 819			$ 230			$ 2,000	$ 816
Ordinary Shares
Number of Shares
Exercised	(2,138)	(14,301)	(3)		(359)	(9,002)
Stock option
Number of Shares
Outstanding number of shares at beginning			863,712				863,712
Granted							2,068,095
Exercised							(16,442)
Forfeited							(278,090)
Outstanding number of shares at end	2,637,275						2,637,275		863,712
Number of shares vested and expected to vest	512,710						512,710
Number of shares options exercisable	512,710						512,710
Weighted Average Exercise Price
Weighted average exercise price at beginning			$ 1.01				$ 1.01
Granted							11.92
Exercised							1.26
Forfeited							3.65
Weighted average exercise price at ending	$ 9.29						9.29		$ 1.01
Weighted average exercise price, vested and expected to vest	5.38						5.38
Weighted average exercise price, options exercisable	$ 5.38						$ 5.38
Weighted Average Contractual Term
Weighted average contractual term outstanding							9 years 3 months 7 days		8 years 9 months
Weighted average contractual term vested and expected to vest							8 years 5 months 1 day
Weighted average contractual term options exercisable							8 years 5 months 1 day
Aggregate Intrinsic Value
Aggregate intrinsic value outstanding	$ 9,088						$ 9,088		$ 3,292
Exercised	100			100			100	$ 100
Aggregate intrinsic value, vested and expected to vest	3,602						3,602
Aggregate intrinsic value, options exercisable	3,602						$ 3,602
Additional Information
Weighted average grant-date fair value of share options granted (per share)							$ 4.81	$ 1.87
Total share-based compensation expense	800			200			$ 1,600	$ 700
Aggregate intrinsic value of share options	100			100			$ 100	$ 100
Performance based option
Number of Shares
Granted							0	70,875
Additional Information
Total share-based compensation expense	$ 31,000			$ 200			$ 100	$ 600
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%