Royalty and other revenue - Additional Information (Detail)	1 Months Ended		3 Months Ended	12 Months Ended
	Aug. 31, 2017 USD ($)	Apr. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Apr. 28, 2017 performance_obligation	Apr. 26, 2017 performance_obligation
License And Royalty Revenue [Line Items]
Cost of license and royalty revenue				$ 5,396,000	$ 2,978,000	$ 885,000
Royalty and other revenue
License And Royalty Revenue [Line Items]
Revenue				21,076,000	8,205,000	2,226,000
Novartis Pharma AG
License And Royalty Revenue [Line Items]
License agreement upfront payment		$ 7,500,000
Revenue recognized upon achievement of related milestone	$ 2,500,000
Novartis Pharma AG | Royalty and other revenue
License And Royalty Revenue [Line Items]
Revenue				21,100,000	8,200,000	2,200,000
Cost of license and royalty revenue				5,400,000	3,000,000.0	900,000
Novartis Pharma AG | Topic 606
License And Royalty Revenue [Line Items]
Number of performance obligation identified at the date of contract inception | performance_obligation								1
Revenue			$ 7,500,000
Novartis Pharma AG | Topic 606 | Regulatory Milestones Payments
License And Royalty Revenue [Line Items]
Revenue			2,500,000
Orchard Therapeutics Limited
License And Royalty Revenue [Line Items]
License agreement upfront payment		3,000,000.0
Orchard Therapeutics Limited | Royalty and other revenue
License And Royalty Revenue [Line Items]
Cost of license and royalty revenue				0	0	0
Orchard Therapeutics Limited | Topic 606
License And Royalty Revenue [Line Items]
Number of performance obligation identified at the date of contract inception | performance_obligation							1
Revenue			$ 3,000,000.0
Number of unsatisfied performance obligation identified at date of contract inception | performance_obligation							0
Orchard Therapeutics Limited | Topic 606 | License
License And Royalty Revenue [Line Items]
Revenue				$ 0	$ 0	$ 0
Regulatory Milestones Payments | Novartis Pharma AG
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		7,500,000
Each Subsequently Licensed Product | Novartis Pharma AG
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		1,100,000
Potential Milestones Payments | Orchard Therapeutics Limited
License And Royalty Revenue [Line Items]
Amount per product eligible to be received upon achievement of specified event		$ 1,300,000