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Share-based compensation - Restricted shares (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted shares
Total share-based compensation expense	$ 6,514	$ 3,083	$ 1,023
Restricted ordinary shares
Restricted shares
Incremental share based compensation expense		200
Total share-based compensation expense	0	400	200
Restricted ordinary shares \| Employees and directors
Restricted shares
Unrecognized compensation cost	$ 0
Restricted ordinary shares \| Employee
Restricted shares
Fair value of employee restricted share awards vested		$ 700	$ 200