Significant agreements - Collaboration revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Significant agreements.
Collaboration revenues	$ 3,848	$ 3,842	$ 1,571	$ 1,129	$ 5,281	$ 614	$ 1,522	$ 6,384	$ 10,390	$ 13,801	$ 7,136
AstraZeneca
Significant agreements.
Collaboration revenues									2,696	1,683	1,386
Sanofi
Significant agreements.
Collaboration revenues									0	10,724	4,007
Oxurion
Significant agreements.
Collaboration revenues									2,362	0	$ 1,743
Dementia Discovery Fund
Significant agreements.
Collaboration revenues									436	394
Material transfer agreement
Significant agreements.
Collaboration revenues									0	$ 1,000
Genentech
Significant agreements.
Collaboration revenues									$ 4,896