Collaborations - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	Jan. 31, 2020	Dec. 31, 2018	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue			$ 1,132,000	$ 1,658,000	$ 2,599,000	$ 18,024,000
Deferred revenue, recognized					1,300
License
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue				23,000		15,068,000
License | Transfer of Exclusive License and Related Know-How
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue						15,000,000.0
Exclusive License
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue						100,000
Collaboration
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue			1,132,000	1,635,000	2,599,000	2,956,000
Collaboration | Reimbursable Out-of-Pocket and Personnel Costs Incurred Related to Research Services
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue			$ 1,100,000	$ 1,600,000	2,600,000	$ 3,000,000.0
ISU Abxis | Amended and Restated License Agreement
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Regulatory and development milestone payment		$ 2,500,000			0
ISU Abxis | Amended and Restated License Agreement | Maximum
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Milestone payment		19,500,000			0
Commercial milestone payments		$ 17,000,000.0			0
Biogen
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue					$ 0
Biogen | License and Collaboration Agreement (the “Biogen Agreement”)
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Research services					30 months
Research services option to extend					12 months
Biogen | Exclusive License
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Up-front payments received	$ 15,000,000.0
Biogen | Exclusive License | Maximum
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Development and sales milestone payments	$ 340,000,000.0