Stock-based compensation - Stock based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Stock options
Stock-based compensation expense included in total operating expenses	$ 5,485	$ 4,039
Research and development
Stock options
Stock-based compensation expense included in total operating expenses	2,933	2,301
General and administrative
Stock options
Stock-based compensation expense included in total operating expenses	2,552	1,738
Stock options
Stock options
Stock-based compensation expense included in total operating expenses	4,118	3,114
Unrecognized stock compensation cost	$ 41,800
Weighted-average amortization period of unrecognized stock compensation cost	2 years 6 months
Restricted stock units
Stock options
Stock-based compensation expense included in total operating expenses	$ 1,209	806
Unrecognized stock compensation cost	$ 13,500
Weighted-average amortization period of unrecognized stock compensation cost	3 years
Employee stock purchase plan
Stock options
Stock-based compensation expense included in total operating expenses	$ 158	$ 119

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

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