Share-based compensation - Share options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Additional Information
Total share-based compensation expense	$ 2,121	$ 276
Ordinary Shares
Number of Shares
Exercised	(1,071)	(3)
Stock option
Number of Shares
Outstanding number of shares at beginning	2,634,346
Granted	1,030,361
Exercised	(1,071)
Forfeited	(8,748)
Outstanding number of shares at end	3,654,888		2,634,346
Number of shares, vested and expected to vest	3,654,888
Number of shares, options exercisable	984,491
Weighted Average Exercise Price
Weighted average exercise price at beginning	$ 9.57
Granted	10.17
Exercised	1.47
Forfeited	8.67
Weighted average exercise price at ending	9.74		$ 9.57
Weighted average exercise price, vested and expected to vest	9.74
Weighted average exercise price, options exercisable	$ 7.71
Weighted Average Contractual Term
Weighted average contractual term, outstanding	9 years 2 months 1 day		9 years 15 days
Weighted average contractual term, vested and expected to vest	9 years 2 months 1 day
Weighted average contractual term ,options exercisable	8 years 9 months 15 days
Aggregate Intrinsic Value
Aggregate intrinsic value outstanding	$ 14,973		$ 2,868
Granted	$ 6.68	$ 3.68
Aggregate intrinsic value, vested and expected to vest	$ 14,973
Aggregate intrinsic value, options exercisable	$ 5,993
Additional Information
Granted	$ 6.68	$ 3.68
Total share-based compensation expense	$ 2,100	$ 200

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Gross number of share options (or share units) granted during the period.

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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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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. 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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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for 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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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Number of share options (or share units) exercised during the current period.

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