Commitments and contingencies - AbbVie Tau Collaboration Agreement - (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2018 USD ($) item	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue		$ 43,689	$ 70,040
Revenue recognized		26,351
AbbVie Tau Collaboration Agreement | AbbVie
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
Number of research antibodies | item	1
Revenue recognized		46,300
Allocation of Transaction Price
Allocation of Transaction Price		69,000
AbbVie Tau Collaboration Agreement | AbbVie | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of development options | item	1
Antibodies to include in Research Period | item	2
The number of Licensed Products | item	1
AbbVie Tau Collaboration Agreement | AbbVie | 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
AbbVie Tau Collaboration Agreement | AbbVie | Research Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized		4,500	$ 11,300	$ 6,900
Allocation of Transaction Price
Allocation of Transaction Price		34,482
AbbVie Tau Collaboration Agreement | AbbVie | First Development Option Material Right
Allocation of Transaction Price
Allocation of Transaction Price		$ 34,518