Share-based compensation - Allocation of Compensation Expense Related to Restricted Shares (Details) - Non-Vested Restricted Shares - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Compensation expense	$ 3,005	$ 1,530	$ 4,774	$ 3,072
Selling, General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Compensation expense	1,839	1,046	3,050	2,114
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Compensation expense	$ 1,166	$ 484	$ 1,724	$ 958