Capital Structure - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Mar. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common Stock reserved for future issuance	43,742,709	42,470,254
Stock Options [Member]
Class of Stock [Line Items]
Common Stock reserved for future issuance	7,475,106	6,200,151
Warrants [Member]
Class of Stock [Line Items]
Common Stock reserved for future issuance	36,267,603	36,270,103