License Agreements - Additional Information (Detail) £ in Thousands, $ in Thousands						1 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 GBP (£)	Oct. 02, 2017 USD ($)	Jan. 27, 2016 USD ($)	Jan. 27, 2016 CAD ($) shares	Aug. 31, 2017 USD ($) shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2018 USD ($)	Nov. 17, 2016 USD ($)
License Agreement [Line Items]
Collaborative agreement expenses							$ 1,437,000	$ 565,000
Common stock, issued | shares							41,569,000	43,652,000
Research and development							$ 83,114,000	$ 87,236,000
Preferred stock issued | shares							0	0
University of Manchester Agreement [Member] | MPSII License Agreement [Member]
License Agreement [Line Items]
Upfront license fee								$ 8,000,000
Milestone payments payable	$ 80,000,000
Milestone payment paid	2,000,000
Milestone payment due	$ 4,000,000
Agreement Expiration Period							15 years
Agreement Expiration Description							upon the later of 15 years from the effective date or the expiration of the last valid claim of the licensed patents
Research and development expense | £		£ 9,900
UHN Agreement [Member]
License Agreement [Line Items]
Research and development expense							$ 209,000	344,000
Milestone fees							0	0
UHN Agreement [Member] | Fabry License Agreement [Member]
License Agreement [Line Items]
Upfront license fee					$ 75
Milestone payments payable					2,450
Option fee					20
UHN Agreement [Member] | Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee					264
Milestone payments payable					19,275
Milestone fees							0	0
Annual maintenance fees					$ 50
Common stock, issued | shares					1,161,665
Fair value of shares issued				$ 480,000
Payments upon closing of an initial public offering				$ 2,000,000					$ 2,000,000
Research and development							39,000	37,000
BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Upfront license fee						$ 500,000
Preferred stock issued | shares						233,765
Milestone payments							13,000,000
Expenses related to license							0	0
GenStem Therapeutics Inc [Member]
License Agreement [Line Items]
Upfront license fee			$ 1,000,000
Milestone payments payable			$ 16,000,000
Expenses related to license							0
Lund University Rights Holders Agreement [Member]
License Agreement [Line Items]
Milestone payments payable										$ 550,000
Expenses related to license							$ 0	$ 0