Commitments and contingencies - Termination of Agreement (Details) - Termination Agreement - Sanofi Genzyme $ in Millions	Jun. 14, 2019 USD ($) item
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of non central nervous system | item	2
Up-front payment made	$ 10.0
Milestone payment agreed to be paid	$ 10.0
Period in which milestone payment will be made of filing of investigational new drug	15 days
Percentage of threshold sublicensing income agreed to be paid	50.00%
Unused in-kind services waived	$ 0.4
Threshold number of capsids having exclusive options to evaluate for no consideration | item	4
Threshold additional number of capsids having options to evaluate for low six-figure payment per capsid | item	2
Threshold number of capsids that has right to obtain an exclusive license | item	2
Option exercise fee	$ 1.0
Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
AAV capsids to license | item	1
Development and regulatory milestones
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 15.0
Sales milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 20.0