Subsequent events (Details) - USD ($) $ in Thousands		2 Months Ended	12 Months Ended
	Oct. 01, 2020	Feb. 28, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2021	Mar. 10, 2021
Subsequent events
Offering expenses			$ 1,900	$ 8,500	$ 327
Subsequent events | Hercules | First Amendment To LSA
Subsequent events
Term loan							$ 15,000
Genentech | Subsequent events
Subsequent events
Sales milestone payments, receivable						$ 2,000
At the market offering
Subsequent events
Shares issued (in shares)			2,928,813
Gross proceeds before discounts and commissions			$ 50,000
Net proceeds			48,100
Offering expenses			$ 1,900
At the market offering | Sales Agreement | Hercules
Subsequent events
Shares issued (in shares)	98,100
Net proceeds	$ 1,800
At the market offering | Sales Agreement | Subsequent events
Subsequent events
Shares sold (in shares)		1,958,485
Gross proceeds before discounts and commissions		$ 50,200
Net proceeds		48,700
Offering expenses		$ 1,500