Quarterly report pursuant to Section 13 or 15(d)

Stockholder's Equity - Schedule of Reserved Shares of Common Stock for Future Issuance (Detail)

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Stockholder's Equity - Schedule of Reserved Shares of Common Stock for Future Issuance (Detail) - shares
Sep. 30, 2018
Dec. 31, 2017
Class of Stock [Line Items]    
Common shares reserved for future issuance 30,405,457 8,076,576
2014 Equity Incentive Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 559,373 492,382
2014 Employee Stock Purchase Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 81,667 83,617
2015 Inducement Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 5,000 5,000
Series C1 Preferred Warrants    
Class of Stock [Line Items]    
Common shares reserved for future issuance 14,033 14,033
June 2016 Public Offering | Warrant liability    
Class of Stock [Line Items]    
Common shares reserved for future issuance 4,218,750 4,218,750
March 2018 Public Offering | Series 1 Warrant    
Class of Stock [Line Items]    
Common shares reserved for future issuance 13,198,075 0
March 2018 Public Offering | Series 2 Warrant    
Class of Stock [Line Items]    
Common shares reserved for future issuance 7,988,175 0
Warrants to purchase common stock associated with Loan Agreement    
Class of Stock [Line Items]    
Common shares reserved for future issuance 122,435 122,435
Stock options    
Class of Stock [Line Items]    
Common shares reserved for future issuance 4,110,108 3,075,994
Restricted Stock Units    
Class of Stock [Line Items]    
Common shares reserved for future issuance 107,841 64,365