License and Research Arrangements - Additional Information (Details) € in Millions	1 Months Ended				3 Months Ended	12 Months Ended
	Apr. 30, 2020 USD ($) shares	May 31, 2019 USD ($)	May 31, 2019 EUR (€)	Apr. 30, 2018 Program	Jun. 30, 2018 USD ($)	Dec. 31, 2022 USD ($) Milestone shares	Dec. 31, 2022 GBP (£) Milestone	Dec. 31, 2016 USD ($) shares	Dec. 31, 2022 GBP (£) shares	Dec. 31, 2022 EUR (€) shares	Dec. 31, 2021 shares	Dec. 31, 2018 shares	Nov. 30, 2018 shares	Sep. 30, 2018 shares	Dec. 31, 2017 shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued | shares						126,947,225			126,947,225	126,947,225	125,674,095
GSK Asset Purchase and License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of additional programs in preclinical development | Program				3
Number of late-stage clinical gene therapy programs | Program				2
Number of earlier stage clinical gene therapy program | Program				1
Number of additional earlier-stage development programs | Program				3
Total consideration					$ 133,600,000
Payment of tiered royalty, maximum percentage						20.00%			20.00%	20.00%
Milestone payments payable upon achievement of certain sales milestones | £									£ 90,000,000.0
Telethon-OSR License Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments payable upon achievement of certain sales milestones						$ 33,400,000				€ 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 ($16.1 million at December 31, 2022) 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.	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 ($16.1 million at December 31, 2022) 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			$ 16,100,000
In-process research and development expense for upfront and milestone payments		$ 17,200,000
Milestone payments payable upon achievement of certain development regulatory and commercial milestones						30,100,000				€ 28.0
UCLB/UCLA Technology
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payments								$ 800,000
UCLB Technology and Manufacturing Technology
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payments								$ 1,100,000
UCLB License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Ordinary shares issued | shares								3,441,290				4,665,384			1,224,094
UCLB/UCLA License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments upon achievement of specified regulatory milestones						$ 24,100,000	£ 19,900,000
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			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	$ 800,000