Significant agreements - Alexion Option and License Agreement (Details) $ in Thousands		3 Months Ended		9 Months Ended
	Oct. 01, 2021 USD ($) item	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenue		$ 4,614	$ 41,086	$ 159,947	$ 42,457
Pfizer | Option and license agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of transgenes | item	2
Number of options | item	1
Number of performance obligations | item	2
Number of material rights | item	2
Upfront payment	$ 30,000
Non-refundable upfront payment received	$ 30,000
Number of license options | item	1
Allocation of transaction price	$ 15,000
Aggregate milestone payments, if exercise rights	$ 10,000
Pfizer | Second Material Right
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of days written notice required by reporting entity to terminate agreement	90 days
Pfizer | Development, regulatory and commercialization milestone | Option and license agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	$ 115,000
Pfizer | Sales milestone | Option and license agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate maximum milestone payments to be received from collaborative partner	$ 175,000
Alexion | Option and license agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized		$ 0	$ 40,000	$ 0	$ 40,000
Minimum | Pfizer | Option and license agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of countries received regulatory approval | item	1
Number of major market countries received regulatory approval | item	1