Quarterly report pursuant to Section 13 or 15(d)

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

v3.20.1
Stockholder's Equity - Schedule of Reserved Shares of Common Stock for Future Issuance (Detail) - shares
Mar. 31, 2020
Dec. 31, 2019
Class of Stock [Line Items]    
Common shares reserved for future issuance 79,278,718 81,439,674
6% Senior Convertible Notes    
Class of Stock [Line Items]    
Common shares reserved for future issuance 11,382,000 11,382,000
2014 Equity Incentive Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 1,900,861 554,774
2014 Employee Stock Purchase Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 81,499 74,231
2015 Inducement Award Plan    
Class of Stock [Line Items]    
Common shares reserved for future issuance 315,500 315,500
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 2 Warrant    
Class of Stock [Line Items]    
Common shares reserved for future issuance 7,988,175 7,988,175
December 2019 Public Offering | Warrant liability    
Class of Stock [Line Items]    
Common shares reserved for future issuance 44,722,222 44,722,222
December 2019 Public Offering | Option to Purchase Common Stock    
Class of Stock [Line Items]    
Common shares reserved for future issuance 0 5,833,333
Warrants to purchase common stock associated with Solar 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 7,684,762 5,261,860
Restricted Stock Units    
Class of Stock [Line Items]    
Common shares reserved for future issuance 862,514 966,394