Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	23,342,390	22,809,913	4,250,049
Class X Convertible Preferred Stock (if-converted) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	11,429,760	11,429,760
Common Stock Options And Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	5,207,374	4,666,359	4,091,701
Warrants for Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	6,682,708	6,682,708	121,512
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	22,548	31,086	36,836