Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Summary of Outstanding Potentially Dilutive Securities Excluded in Calculation of Diluted Net Loss Per Share (Detail)

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Summary of Significant Accounting Policies - Summary of Outstanding Potentially Dilutive Securities Excluded in Calculation of Diluted Net Loss Per Share (Detail) - shares
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 7,364,913 3,660,261
ESPP shares pending issuance [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 7,514 7,415
Warrants to purchase common stock [Member] | Common Stock [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 149,704 149,704
Common stock options issued and outstanding [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 4,910,557 3,496,826
Shares issuable upon conversion of convertible note payable [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 2,297,138 0
Common stock subject to repurchase [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 0 6,316