Significant agreements - Dementia Discovery Fund Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Significant agreements.
Collaboration revenues		$ 3,842	$ 614	$ 6,542	$ 8,520
Deferred revenue		33,275		33,275		$ 5,657	$ 14,635
Dementia Discovery Fund
Significant agreements.
Upfront cash payment	$ 1,100
Potential milestone payments	$ 700
Threshold period for exercising option to establish a jointly-owned new company	90 days
Option to purchase ownership of new entity	34.00%
Threshold period of notice required for termination of agreement	60 days
Transaction price	$ 1,100
Collaboration revenues		58	$ 173	170	$ 275
Deferred revenue		$ 550		$ 550		$ 744
Dementia Discovery Fund | NewCo
Significant agreements.
Option to purchase ownership of new entity	66.00%
Threshold period in which Newco shall have the right to initiate a new research program	2 years
Dementia Discovery Fund | NewCo | Maximum
Significant agreements.
Total voting rights related to ownership interests	50.00%