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
Jun. 30, 2019
Dec. 31, 2018
Class of Stock [Line Items]    
Common shares reserved for future issuance 30,930,040 30,404,957
6.0% Convertible Senior Notes due 2025    
Class of Stock [Line Items]    
Common shares reserved for future issuance 11,382,000 0
2014 Equity Incentive Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 265,156 612,018
2014 Employee Stock Purchase Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 91,819 81,667
2015 Inducement Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 315,500 5,000
Series C1 Preferred Warrants    
Class of Stock [Line Items]    
Common shares reserved for future issuance 0 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 0 13,198,075
March 2018 Public Offering | Series 2 Warrant    
Class of Stock [Line Items]    
Common shares reserved for future issuance 7,988,175 7,988,175
Warrants to purchase common stock associated with Loan Agreement | Warrant liability    
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 5,578,978 4,052,913
Restricted Stock Units    
Class of Stock [Line Items]    
Common shares reserved for future issuance 967,227 111,891