Stock-based compensation - Compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	$ 2,490	$ 3,991	$ 4,809	$ 7,592
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	1,618	2,480	3,095	5,002
Unvested restricted common stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	842	1,392	1,634	2,366
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	30	120	81	223
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	781	1,336	1,582	2,706
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	$ 1,709	$ 2,655	$ 3,227	$ 4,886