Collaborations - Additional Information (Detail) - USD ($) $ in Thousands		1 Months Ended			3 Months Ended	9 Months Ended
	Dec. 18, 2019	Jan. 31, 2020	Dec. 31, 2018	Feb. 28, 2018	Sep. 30, 2020	Sep. 30, 2020	Dec. 08, 2016
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue					$ 893	$ 18,917
License
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue					32	15,100
License | Transfer of Exclusive License and Related Know-How
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue						15,000
Exclusive License
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue						100
Collaboration
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue					861	3,817
Collaboration | Reimbursable Out-of-Pocket and Personnel Costs Incurred Related to Research Services
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
License and collaboration revenue					$ 900	3,800
Pfizer Inc | Collaboration Agreement
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Milestone payment				$ 1,000		0
Pfizer Inc | Collaboration Agreement | Maximum
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Contingent cash payments upon achievement of certain clinical, regulatory and commercial milestones							$ 17,500
ISU Abxis | Amended and Restated License Agreement
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Regulatory and development milestone payment			$ 2,500			0
ISU Abxis | Amended and Restated License Agreement | Maximum
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Milestone payment			19,500			0
Commercial milestone payments			$ 17,000			$ 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
Biogen | Exclusive License | Maximum
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Development and sales milestone payments		$ 340,000