Capital Structure - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Sep. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common Stock reserved for future issuance	38,513,380	42,470,254
Stock Options [Member]
Class of Stock [Line Items]
Common Stock reserved for future issuance	7,248,198	6,200,151
Warrants [Member]
Class of Stock [Line Items]
Common Stock reserved for future issuance	31,265,182	36,270,103