Stock-based compensation - Compensation expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	$ 14,934	$ 15,640	$ 15,710
Outstanding stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	11,387	13,380	14,956
Unvested restricted common stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	3,110	1,935	482
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	437	325	272
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	6,357	7,383	4,717
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.	$ 8,577	8,257	10,993
Dr. Sah
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.		$ 2,200
Consultant duration	3 years
Dr. Paul
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation.			$ 5,400
Consultant duration	3 years