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Commitments and Contingencies - Additional Information (Detail) | 1 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Feb. 28, 2018 USD ($) | Nov. 30, 2017 ft² | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2013 USD ($) | Feb. 21, 2018 USD ($) | May 20, 2016 USD ($)
Commitments And Contingencies Disclosure [Line Items] |  |  |  |  |  |  |
Operating lease expiration date |  |  | 2018-02 |  |  |  |
Operating lease rent expense |  |  | $ 800,000 | $ 700,000 |  |  |
License fees payable |  |  |  |  | $ 100,000 |  |
AGC Biologics, Inc |  |  |  |  |  |  |
Commitments And Contingencies Disclosure [Line Items] |  |  |  |  |  |  |
Contractual obligation payments |  |  | 700,000 |  |  |  | $ 3,800,000
AGC Biologics, Inc | Subsequent Event |  |  |  |  |  |  |
Commitments And Contingencies Disclosure [Line Items] |  |  |  |  |  |  |
Contractual obligation payments |  |  |  |  |  | $ 5,600,000 |
ISU Abxis | Maximum |  |  |  |  |  |  |
Commitments And Contingencies Disclosure [Line Items] |  |  |  |  |  |  |
Potential milestone payments |  |  | 2,000,000 |  |  |  |
Pfizer Inc | Collaboration Agreement | Maximum |  |  |  |  |  |  |
Commitments And Contingencies Disclosure [Line Items] |  |  |  |  |  |  |
Obligated to make milestone and royalty payments upon achievement of clinical, regulatory and commercial milestones |  |  | $ 17,500,000 |  |  |  |
Pfizer Inc | Subsequent Event | Collaboration Agreement |  |  |  |  |  |  |
Commitments And Contingencies Disclosure [Line Items] |  |  |  |  |  |  |
Milestone payment | $ 1,000,000 |  |  |  |  |  |
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 |  |  |  |  |

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