Significant agreements - Dementia Discovery Fund Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Significant agreements
Collaboration revenues		$ 1,522	$ 1,661	$ 7,906	$ 4,469
Deferred revenue		10,370		10,370		$ 14,635	$ 14,467
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		103	$ 0	103	$ 0
Deferred revenue		$ 1,012		$ 1,012		$ 0
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%