Significant agreements - Dementia Discovery Fund Agreement (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended								12 Months Ended
	Dec. 31, 2020	May 31, 2019	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
Deferred revenue	$ 35,156		35,156				5,657				35,156	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											436	394
Deferred revenue	821		$ 821				$ 744				$ 821	$ 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%
Specific scientific criteria milestone | Dementia Discovery Fund
Significant agreements.
Milestone payment received	$ 500