Commitments and Contingencies - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018 USD ($) ft²	Dec. 08, 2016 USD ($)	May 20, 2016 USD ($)
Commitments And Contingencies Disclosure [Line Items]
Operating lease expiration date	2018-02
South San Francisco, California | New Office Lease Agreement
Commitments And Contingencies Disclosure [Line Items]
Leased area | ft²	8,606
Operating lease term of contract	5 years 2 months
Operating lease commencement date	Feb. 16, 2018
Pfizer Inc | Collaboration Agreement
Commitments And Contingencies Disclosure [Line Items]
Milestone payment	$ 1,000,000
Pfizer Inc | Collaboration Agreement | Maximum
Commitments And Contingencies Disclosure [Line Items]
Contingent cash payments upon achievement of clinical, regulatory and commercial milestones		$ 17,500,000
AGC Biologics, Inc | MarzAA
Commitments And Contingencies Disclosure [Line Items]
Contractual obligation payments	500,000		$ 3,800,000
Accrued liabilities	400,000
AGC Biologics, Inc | CB 2679d/ISU 304
Commitments And Contingencies Disclosure [Line Items]
Contractual obligation payments	$ 4,100,000		$ 5,600,000