Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Total shares of common stock reserved	14,209,402	10,822,328
Shares available for future stock option grants
Class Of Stock [Line Items]
Total shares of common stock reserved	969,783	2,168,391
ESPP shares available for issuance
Class Of Stock [Line Items]
Total shares of common stock reserved	1,103,481	667,670
Warrant to purchase common stock
Class Of Stock [Line Items]
Total shares of common stock reserved	3,393,755	1,707,736
Options To Purchase Common Stock
Class Of Stock [Line Items]
Total shares of common stock reserved	8,592,747	6,127,291
Shares issuable upon settlement of restricted stock units outstanding
Class Of Stock [Line Items]
Total shares of common stock reserved	149,636	151,240