Commitments and contingencies - AbbVie Alpha Synuclein Collaboration Agreement - (Details) - AbbVie Alpha Synuclein Collaboration Agreement - AbbVie $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Feb. 28, 2019 USD ($) item	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) item
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of antibodies encoded by virus | item	1
Number of research antibodies | item	1
Non-refundable upfront payment received	$ 65,000
First Research Compound payment	80,000
Three additional research compounds payment to be received for each	$ 30,000
Number of additional research compounds | item	3
Milestone payments to be received for licensed compound in case Parkinson's disease indication	$ 450,000
Milestone payments to be received for licensed compound in case first indication other than Parkinson's disease	185,000
Milestone payments to be received for licensed compound for subsequent non-Parkinson's disease indication	$ 92,500
Binds to alpha-synuclein protein, after the termination date (in months)	18 months
Number of development options | item			4
Deferred revenue		$ 64,300	$ 64,300
Revenue recognized		$ 200	$ 700
Allocation of Transaction Price
Allocation of Transaction Price	$ 65,000
Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Antibodies to include in Research Period | item	1
The number of Licensed Products | item	1
Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Antibodies to include in Research Period | item	4
Research Services
Allocation of Transaction Price
Allocation of Transaction Price	$ 23,768
First Development Option Material Right
Allocation of Transaction Price
Allocation of Transaction Price	41,232
Commercial Milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Specified commercial milestone payments	$ 500,000