Commitments and contingencies - AbbVie Alpha Synuclein Collaboration Agreement - (Details) - AbbVie Alpha Synuclein Collaboration Agreement - AbbVie $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2019 USD ($) item	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) item	Jun. 30, 2019 USD ($)
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
Number of development options | item				4
Deferred revenue		$ 59,400		$ 59,400
Revenue recognized		$ 3,100	$ 200	$ 4,300	$ 400
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