Significant agreements - Collaboration revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Significant agreements.
Collaboration revenues	$ 4,333	$ 3,842	$ 7,926	$ 6,542
Ionis
Significant agreements.
Collaboration revenues	2,066	0	2,066	0
Genentech
Significant agreements.
Collaboration revenues	1,063	1,209	4,142	3,066
Dementia Discovery Fund
Significant agreements.
Collaboration revenues	77	58	314	170
AstraZeneca
Significant agreements.
Collaboration revenues	1,127	213	1,404	944
Oxurion
Significant agreements.
Collaboration revenues	$ 0	$ 2,362	$ 0	$ 2,362	$ 0