Preferred Stock, Common Stock and Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock options issued and outstanding	2,131,624	1,275,624
Authorized for future option grants	72,801	628,801
Warrants to purchase common stock	2,797,561	3,323,876
Authorized for employee stock purchase plan	91,934	37,463
Total common stock reserved for future issuance	5,093,920	5,265,764