License Agreements - Additional Information (Detail) $ in Thousands					1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Oct. 02, 2017 USD ($)	Nov. 17, 2016 USD ($)	Jan. 27, 2016 USD ($) shares	Jan. 27, 2016 CAD ($)	Aug. 31, 2017 USD ($) shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 shares	Jan. 27, 2016 CAD ($) shares
License Agreement [Line Items]
Common stock, shares issued | shares						24,187,052		24,187,052			23,959,903
Fair value of shares issued							$ 104,182,000
Research and development						$ 12,267,000	7,407,000	$ 24,713,000	$ 13,054,000
Preferred stock issued | shares						0		0			0
UHN Agreement [Member]
License Agreement [Line Items]
Milestone fees								$ 0	0
Annual maintenance fees								0	0
Research and development expense						$ 132,000	0	325,000	0
UHN Agreement [Member] | Fabry License Agreement [Member]
License Agreement [Line Items]
Option fee												$ 20
Upfront license fee				$ 75
Milestone payments				2,450
UHN Agreement [Member] | Interleukin 12 License Agreement [Member]
License Agreement [Line Items]
Upfront license fee				264
Milestone payments				19,275
Annual maintenance fees				$ 50
Common stock, shares issued | shares			1,161,665									1,161,665
Fair value of shares issued			$ 480
Payments upon closing of an initial public offering			2,000,000
Fair value of derivative liability			$ 49,000
Research and development						$ 38,000	$ 41,000	38,000	41,000
BioMarin Pharmaceutical Inc [Member]
License Agreement [Line Items]
Upfront license fee					$ 500,000
Milestone payments								13,000,000
Upfront cash payment										$ 500,000
Expenses related to license								0	0
BioMarin Pharmaceutical Inc [Member] | Series B Preferred Stock [Member]
License Agreement [Line Items]
Research and development										$ 500,000
Preferred stock issued | shares					233,765
GenStem Therapeutics Inc [Member]
License Agreement [Line Items]
Upfront license fee	$ 1,000,000
Expenses related to license								0	0
Milestone payments	16,000,000
Milestone payment due	$ 2,000,000
Lund University Rights Holders Agreement [Member]
License Agreement [Line Items]
Expenses related to license								$ 0	$ 0
Milestone payments		$ 550