Quarterly report pursuant to Section 13 or 15(d)

Commitments and Contingencies - Additional Information (Detail)

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Commitments and Contingencies - Additional Information (Detail)
3 Months Ended 9 Months Ended
Feb. 15, 2018
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Feb. 28, 2017
ft²
Loss Contingencies [Line Items]              
Accrued termination charges   $ 0   $ 0   $ 0  
Letter of credit issued   1,400,000   1,400,000   1,400,000  
Rental expenses   600,000 $ 400,000 1,600,000 $ 1,000,000    
Letter of credit issued, classified as long-term restricted cash   1,200,000   1,200,000   1,200,000  
Liabilities related to indemnification agreements   0   $ 0   $ 0  
Minimum              
Loss Contingencies [Line Items]              
Lease agreement term       2 years      
Maximum              
Loss Contingencies [Line Items]              
Lease agreement term       3 years      
South San Francisco California Lease              
Loss Contingencies [Line Items]              
Lease expiration date       Apr. 30, 2021      
Letter of credit renewal term       12 months      
South San Francisco California Lease | Letter of Credit              
Loss Contingencies [Line Items]              
Letter of credit issued   200,000   $ 200,000      
Westlake Village California Lease              
Loss Contingencies [Line Items]              
Lease expiration date       Apr. 30, 2019      
Thousand Oaks California Lease              
Loss Contingencies [Line Items]              
Lease agreement area of office, lab and cellular therapy manufacturing space | ft²             90,580
Lease initial term 15 years            
Lease commencement date       Feb. 15, 2018      
Contractual obligations $ 16,400,000            
Lease extension term, option one 10 years            
Lease extension term, option two 9 years            
Thousand Oaks California Lease | Letter of Credit              
Loss Contingencies [Line Items]              
Letter of credit issued, classified as long-term restricted cash   1,200,000   $ 1,200,000      
Build To Suit Lease Arrangement | Building              
Loss Contingencies [Line Items]              
Construction in progress   $ 10,300,000   $ 10,300,000