Significant agreements - Novartis Option and License Agreement (Details) $ in Thousands			1 Months Ended	3 Months Ended				9 Months Ended
	Mar. 01, 2023 USD ($) item	Mar. 04, 2022 USD ($) item	Apr. 30, 2023 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($) item	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenue | $				$ 4,614			$ 41,086	$ 159,947	$ 42,457
Related party collaboration receivable | $				$ 3,253				$ 3,253		$ 257
Novartis Pharma, AG | Novartis License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of transgenes		3
Number of additional targets	2	2
Upfront payment | $	$ 54,000	$ 54,000
Additional fee agreed to pay | $	$ 18,000
Number of royalties	2
Exercise payment | $			$ 25,000
Number of targets not elected to license a capsid	1
Aggregate milestone payments, if exercise rights | $	$ 12,500
Number of license option exercise for each target		1
Number of Alliance Managers		2
Number of options		1
Number of major market countries received regulatory approval		3
Number of days written notice required by reporting entity to terminate agreement	90 days
Number of performance obligations		3
Number of material rights		3				2
Allocation of transaction price | $		$ 18,000
Revenue recognized | $					$ 25,000	$ 79,000
Novartis Pharma, AG | Novartis License Agreement | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of license options		3
Novartis Pharma, AG | Novartis License Agreement | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of license options		5
Novartis Pharma, AG | Material Right | Novartis License Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized | $						$ 54,000