Significant agreements - Dementia Discovery Fund Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Significant agreements.
Collaboration revenues		$ 1,571	$ 1,522	$ 2,700	$ 7,906
Deferred revenue		33,082		33,082		$ 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		56	$ 103	112	$ 103
Deferred revenue		$ 582		$ 582		$ 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%