Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Total shares of common stock reserved	21,576,339	14,209,402
Shares available for future stock option grants
Class Of Stock [Line Items]
Total shares of common stock reserved	1,908,626	969,783
ESPP shares available for issuance
Class Of Stock [Line Items]
Total shares of common stock reserved	1,780,020	1,103,481
Warrants to purchase common stock
Class Of Stock [Line Items]
Total shares of common stock reserved	4,426,244	3,393,755
Options To Purchase Common Stock
Class Of Stock [Line Items]
Total shares of common stock reserved	11,603,708	8,592,747
Shares issuable upon settlement of restricted stock units outstanding
Class Of Stock [Line Items]
Total shares of common stock reserved	1,857,741	149,636