Significant agreements - Oxurion Collaboration Agreement (Details) $ in Thousands, € in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 EUR (€)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Significant agreements.
Revenue from collaborative arrangement | $	$ 3,848	$ 3,842	$ 1,571	$ 1,129	$ 5,281	$ 614	$ 1,522	$ 6,384		$ 10,390	$ 13,801	$ 7,136
Oxurion
Significant agreements.
Threshold period of notice required for termination of agreement									90 days	90 days
Revenue recognized | $										$ 2,400		1,200
Revenue from collaborative arrangement | $										$ 2,362	$ 0	$ 1,743
Oxurion | Development milestone
Significant agreements.
Potential milestone payments									€ 8.3
Revenue under contracts with customer from milestone payment									1.8
Oxurion | Regulatory milestone
Significant agreements.
Potential milestone payments									16.5
Oxurion | Granting first regulatory approval or first indication
Significant agreements.
Potential milestone payments									€ 5.0