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Stockholders' Equity - Schedule of Reserved Shares of Common Stock for Future Issuance (Detail) - shares
Mar. 31, 2021
Dec. 31, 2020
Class of Stock [Line Items]    
Common shares reserved for future issuance 27,510,704 28,028,978
Stock options    
Class of Stock [Line Items]    
Common shares reserved for future issuance 1,371,606 830,343
Restricted Stock Units    
Class of Stock [Line Items]    
Common shares reserved for future issuance 96,974 29,087
Warrant | June 2016 Public Offering    
Class of Stock [Line Items]    
Common shares reserved for future issuance 421,867 421,867
Warrant | December 2019 Public Offering    
Class of Stock [Line Items]    
Common shares reserved for future issuance 4,472,205 4,472,205
Warrant | December 2020 Public Offering Series 1    
Class of Stock [Line Items]    
Common shares reserved for future issuance 6,800,000 6,800,000
Warrant | December 2020 Public Offering Series 2    
Class of Stock [Line Items]    
Common shares reserved for future issuance 6,800,000 6,800,000
Warrant | Prefunded Warrants to Purchase Common Stock Associated with December 2020 Public Offering    
Class of Stock [Line Items]    
Common shares reserved for future issuance 5,260,000 5,260,000
Warrant | Warrants to purchase common stock associated with Solar loan agreement    
Class of Stock [Line Items]    
Common shares reserved for future issuance 12,243 12,243
Series 1 Warrant | March 2018 Public Offering    
Class of Stock [Line Items]    
Common shares reserved for future issuance 798,810 798,810
March 2019 Notes    
Class of Stock [Line Items]    
Common shares reserved for future issuance 1,138,200 1,138,200
April 2020 Notes    
Class of Stock [Line Items]    
Common shares reserved for future issuance 0 1,299,790
2014 Equity Incentive Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 318,097 146,488
2014 Employee Stock Purchase Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 6,652 5,895
2015 Inducement Award Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 14,050 14,050