Stock-based compensation - Compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	$ 3,601	$ 4,040
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	2,523	3,208
Unvested restricted common stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	975	741
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	103	91
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	1,371	1,685
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	$ 2,230	$ 2,355