License Agreements - Additional Information (Details) € in Millions, £ in Millions, $ in Millions	1 Months Ended				3 Months Ended		6 Months Ended
	Apr. 30, 2020 USD ($) shares	May 31, 2019 USD ($)	May 31, 2019 EUR (€)	Apr. 30, 2018 Program	Jun. 30, 2022 USD ($) Milestone shares	Jun. 30, 2018 USD ($)	Jun. 30, 2022 USD ($) Milestone shares	Jun. 30, 2022 GBP (£) shares	Jun. 30, 2022 EUR (€) shares	Dec. 31, 2021 shares	Nov. 30, 2018 shares	Sep. 30, 2018 shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary Shares, Issued | shares					126,436,213		126,436,213	126,436,213	126,436,213	125,674,095
GSK Asset Purchase and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of additional programs in preclinical development				3
Number of late-stage clinical gene therapy programs				2
Number of earlier stage clinical gene therapy program				1
Number of additional earlier-stage development programs				3
Total consideration | $						$ 133.6
Payment of tiered royalty, maximum percentage					20.00%		20.00%	20.00%	20.00%
Milestone payments payable upon achievement of certain sales milestones | £								£ 90.0
Telethon-OSR License Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments payable upon achievement of certain sales milestones					$ 32.4		$ 32.4		€ 31.0
Upfront payment description							In May 2019, the Company entered into a license agreement with Telethon-OSR, under which Telethon-OSR granted to the Company an exclusive worldwide license for the research, development, manufacture and commercialization of Telethon-OSR’s ex vivo autologous HSC lentiviral based gene therapy for the treatment of mucopolysaccharidosis type I, including the Hurler variant (“MPS-IH”). Under the terms of the agreement, Telethon-OSR received €15.0 million in upfront and milestone payments from the Company upon entering into the agreement, resulting in $17.2 million in in-process research and development expense.
Upfront and milestone payments | €			€ 15.0
In-process research and development expense for upfront and milestone payments | $		$ 17.2
Milestone payments payable upon achievement of certain development regulatory and commercial milestones					$ 29.3		$ 29.3		€ 28.0
Oxford BioMedica License, Development and Supply Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary Shares, Issued | shares	75,413				588,220		588,220	588,220	588,220		150,826	150,826
Number of milestones met | Milestone					0		0
Oxford BioMedica License, Development and Supply Agreement | Research and Development Expense
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment | $	$ 0.8