Commitments and contingencies - Neurocrine Collaboration Agreement - (Details) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) shares	Mar. 31, 2019 USD ($) item $ / shares shares	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Allocation of Transaction Price
Revenue recognized				$ 26,351
Deferred revenue			$ 43,689	43,689	$ 70,040
Related party collaboration receivable			8,012	8,012	18,496
Neurocrine | FA Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		60.00%
Neurocrine Collaboration Agreement
Allocation of Transaction Price
Related party collaboration receivable			8,000	8,000	18,500
Neurocrine Collaboration Agreement | VY-AADC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		50.00%
Percentage of profit or loss under co-co option after paying rate-shifting fee		45.00%
Neurocrine Collaboration Agreement | FA Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		40.00%
Neurocrine Collaboration Agreement | Neurocrine
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of collaboration programs | item		4
Number of discovery programs | item		2
Programs added later | item		2
Number of targets | item		8
Upfront payment	$ 115,000	$ 115,000
Purchase of common stock, shares | shares	4,179,728	4,179,728
Purchase of common stock	$ 5,000	$ 50,000
Price per share | $ / shares		$ 11.9625
Aggregate maximum milestone payments to be received from collaborative partner		$ 170,000
Estimated cost reimbursement	431,100		316,200
Termination period		10 years
Period of advance notice for termination prior to first commercial sale		180 days
Period of advance notice for termination after first commercial sale		1 year
Discount related to equity investment	27,600
Equity investment	50,000
Allocation of Transaction Price
Allocation of variable consideration			316,233	316,233
Allocation of fixed consideration	92,400		92,382	92,382
Revenue recognized				56,700	$ 60,000
Deferred revenue			43,800	43,800
Reimbursement costs expected to be received				8,000
Costs to obtain collaboration agreement			800	800
Consideration received	$ 5,000
Neurocrine Collaboration Agreement | Neurocrine | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of reduction in royalty payments		50.00%
Neurocrine Collaboration Agreement | Neurocrine | VY-AADC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Percentage of profit or loss under co-co option		50.00%
Rate-shifting fee		$ 35,000
Percentage of profit or loss under co-co option after paying rate-shifting fee		55.00%
Allocation of Transaction Price
Allocation of variable consideration			86,480	86,480
Allocation of fixed consideration			80,373	80,373
Neurocrine Collaboration Agreement | Neurocrine | FA Program
Allocation of Transaction Price
Allocation of variable consideration			87,991	87,991
Allocation of fixed consideration			6,005	6,005
Neurocrine Collaboration Agreement | Neurocrine | Discovery program 1
Allocation of Transaction Price
Allocation of variable consideration			72,247	72,247
Allocation of fixed consideration			3,002	3,002
Neurocrine Collaboration Agreement | Neurocrine | Discovery program 2
Allocation of Transaction Price
Allocation of variable consideration			69,515	69,515
Allocation of fixed consideration			$ 3,002	$ 3,002
Neurocrine Collaboration Agreement | Neurocrine | Development and regulatory milestones | FA Program
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner		$ 195,000
Neurocrine Collaboration Agreement | Neurocrine | Development and regulatory milestones | Discovery program 1
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner		130,000
Neurocrine Collaboration Agreement | Neurocrine | Commercial Milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner		275,000
Aggregate maximum milestone payments to be received from collaborative partner		$ 1,100,000