Net Loss Per Share - Schedule of Outstanding Shares of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	10,577,941	7,470,601	6,543,654
Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	10,504,412	7,470,601	6,543,654
Warrants for Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	73,529