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Collaborations - Additional Information (Detail) - USD ($)		1 Months Ended		12 Months Ended
	Sep. 16, 2013	Dec. 31, 2018	Feb. 28, 2018	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018	Dec. 08, 2016
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Contract revenue				$ 6,000	$ 1,018,000
Milestone payment received				1,500,000
Cumulative adjustment to accumulated deficit		$ (203,342,000)		(203,342,000)	(173,494,000)
Pfizer Inc | Collaboration Agreement
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Milestone payment			$ 1,000,000
Pfizer Inc | Collaboration Agreement | Maximum
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Contingent cash payments upon achievement of clinical, regulatory and commercial milestones							$ 17,500,000
ISU Abxis
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Contract revenue				0	300,000
Milestone payment received				0	900,000
Deferred revenue		0		$ 0	$ 200,000
ISU Abxis | Up Front Payment
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Amount received for research and development process	$ 1,750,000
ISU Abxis | Difference between Revenue Guidance in Effect before and after Topic 606 | ASU 2014-09
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Cumulative adjustment to accumulated deficit						$ 200,000
ISU Abxis | Amended and Restated License Agreement
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Regulatory and development milestone payment		2,500,000
ISU Abxis | Amended and Restated License Agreement | Maximum
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Milestone payment		19,500,000
Commercial milestone payments		$ 17,000,000

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