Significant agreements - Collaboration revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant agreements.
Collaboration revenues	$ 5,281	$ 614	$ 1,522	$ 6,384	$ 1,057	$ 1,610	$ 1,661	$ 2,808	$ 13,801	$ 7,136	$ 2,060
AstraZeneca
Significant agreements.
Collaboration revenues									1,683	1,386	890
Sanofi
Significant agreements.
Collaboration revenues									10,724	4,007	355
Oxurion
Significant agreements.
Collaboration revenues									0	1,743	$ 815
Dementia Discovery Fund
Significant agreements.
Collaboration revenues									394
Material transfer agreement
Significant agreements.
Collaboration revenues									$ 1,000	$ 0