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
shares in Thousands
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 8,393 6,948
ESPP shares pending issuance [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 6 4
Warrants to purchase common stock [Member] | Common Stock [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 13 150
Common stock options issued and outstanding [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 5,528 4,329
Shares issuable upon conversion of convertible note payable [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding potentially dilutive securities 2,846 2,465