Quarterly report pursuant to Section 13 or 15(d)

Employee Stock Purchase Plan and Stock-Based Compensation - Additional Information (Details)

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Employee Stock Purchase Plan and Stock-Based Compensation - Additional Information (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 26, 2018
Sep. 25, 2018
Feb. 28, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Common stock reserved for issuance       22,107,783   22,107,783    
Aggregate intrinsic value of stock options exercised       $ 18,275 $ 8,431 $ 153,414 $ 51,107  
Weighted-average estimated grant-date fair value of employee stock options granted       $ 8.93 $ 7.20 $ 8.93 $ 7.14  
Total fair value of stock options vested       $ 300,000 $ 600,000 $ 700,000 $ 1,400,000  
Stock-based compensation expense       321 593 802 1,126  
Other income (expense), net       (68,000) (180,000) 840,000 (197,000)  
Non-employees                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Stock-based compensation expense       1,019 19,055 20,795 58,212  
ESPP                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Common stock reserved for issuance 230,000              
Annual increase period of common stock reserved for issuance 10 years              
Maximum number of shares issuable 2,300,000              
ESPP | Maximum                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Increase in stock reserved for issuance as percentage of capital stock outstanding on last day of preceding year 1.00%              
2018 Equity Incentive Plan                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Common stock reserved for issuance   2,300,000            
Increase in stock reserved for issuance as percentage of capital stock outstanding on last day of preceding year   5.00%            
2018 Equity Incentive Plan | Maximum                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Annual increase period of common stock reserved for issuance   10 years            
2004 and 2018 Equity Incentive Plan                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Total unrecognized compensation cost related to unvested stock options granted       22,200,000   $ 22,200,000    
Remaining unrecognized compensation cost expected to be recognized over weighted-average period           3 years 7 months 6 days    
2004 and 2018 Equity Incentive Plan | Restricted Stock                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Share-based compensation arrangement by share-based payment right to repurchase of shares lapse with respect to share percentage after year one           25.00%    
Share-based compensation arrangement by share-based payment award ratable lapse remainder period           3 years    
2004 and 2018 Equity Incentive Plan | Other Noncurrent Liabilities | Restricted Stock                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Cash received for early exercise of options       0   $ 0   $ 14,000
2017 Call Option Plan                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Stock-based compensation expense       $ 19,000 15,000 $ 50,000 65,000  
Call options expiration period     1 year          
Share-based compensation arrangement by share-based payment award options, outstanding number of shares       315,000   315,000    
Share-based compensation arrangement by share-based payment award options, unvested shares       210,000   210,000    
Share-based compensation arrangement by share-based payment award options, shares granted               420,000
Share-based compensation arrangement by share based payment award options, exercise price per share               $ 0.76
Share-based compensation arrangement by share-based payment award options vested number of shares           105,000   105,000
Share-based compensation arrangement by share-based payment award options exercised shares           0   105,000
Other income (expense), net       $ 6,000 $ 6,000 $ 10,000 $ 10,000  
2017 Call Option Plan | SutroVax                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Common stock reserved for issuance       450,000   450,000    
2017 Call Option Plan | January 1 , 2017                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Call options vesting percentage     25.00%          
2017 Call Option Plan | January 1 , 2018                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Call options vesting percentage     25.00%          
2017 Call Option Plan | January 1 , 2019                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Call options vesting percentage     25.00%          
2017 Call Option Plan | January 1 , 2020                
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]                
Call options vesting percentage     25.00%