Share-based compensation - Share options (Details) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Additional Information
Total share-based compensation expense	$ 3,083,000	$ 1,023,000	$ 515,000
Ordinary Shares
Number of Shares
Exercised	(85,839)	(9,361)	(4,300)
Stock option
Number of Shares
Outstanding number of shares at beginning	863,712
Granted	2,134,538
Exercised	(85,839)
Forfeited	(278,065)
Outstanding number of shares at end	2,634,346	863,712
Number of shares, vested and expected to vest	2,634,346
Number of shares, options exercisable	647,901
Weighted Average Exercise Price
Weighted average exercise price at beginning	$ 1.01
Granted	12.01
Exercised	1.67
Forfeited	4.21
Weighted average exercise price at ending	9.57	$ 1.01
Weighted average exercise price, vested and expected to vest	9.57
Weighted average exercise price, options exercisable	$ 6.81
Weighted Average Contractual Term
Weighted average contractual term, outstanding	9 years 15 days	8 years 9 months
Weighted average contractual term, vested and expected to vest	9 years 15 days
Weighted average contractual term ,options exercisable	8 years 5 months 27 days
Aggregate Intrinsic Value
Aggregate intrinsic value outstanding	$ 6,107,000	$ 3,292,000
Granted	$ 6.07	$ 3.73	$ 1.78
Exercised	$ 600,000	$ 23,000	$ 7,000
Aggregate intrinsic value, vested and expected to vest	6,107,000
Aggregate intrinsic value, options exercisable	$ 2,868,000
Additional Information
Granted	$ 6.07	$ 3.73	$ 1.78
Total share-based compensation expense	$ 2,700,000	$ 800,000	$ 400,000
Performance based option
Number of Shares
Granted	0	70,875	678,610
Additional Information
Total share-based compensation expense	$ 100,000	$ 700,000	$ 300,000
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%

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Period for vesting equity-based award in equal monthly installment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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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 price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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

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