Commitments and contingencies - AbbVie Tau Collaboration Agreement - (Details) - AbbVie Tau Collaboration Agreement - AbbVie $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2018 USD ($) item	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-refundable upfront payment received	$ 69,000
First Research Compound payment	80,000
Two additional research compounds payment to be received for each	$ 30,000
Number of additional research compounds | item	2
One-time payment upon exercise of license option	$ 75,000
Milestone payments to be received for licensed compound in case Alzheimer disease indication	550,000
Milestone payments to be received for licensed compound in case first indication other than Alzheimer disease	230,000
Milestone payments to be received for licensed compound for subsequent non-Alzheimer disease indication	$ 115,000
Number of development options | item	3			3
Number of research antibodies | item	1
Deferred revenue		$ 54,000		$ 54,000
Revenue recognized		3,200	$ 2,000	8,100	$ 4,300
Allocation of Transaction Price
Allocation of Transaction Price		69,000		69,000
Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Antibodies to include in Research Period | item	2
The number of Licensed Products | item	1
Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of antibodies encoded by virus | item	5
Antibodies to include in Research Period | item	3
Research Services
Allocation of Transaction Price
Allocation of Transaction Price		34,482		34,482
First Development Option Material Right
Allocation of Transaction Price
Allocation of Transaction Price		$ 34,518		$ 34,518