Stockholders' Equity - Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants, Employee Stock Purchase Plan and Performance Options with Market Condition (Detail) - Employee Stock Option [Member]	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	2.30%	2.00%
Risk-free interest rate, maximum	2.70%	2.10%
Expected volatility, minimum	89.20%	104.00%
Expected volatility, maximum	98.40%	105.20%
Expected dividend yield	0.00%	0.00%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 9 months 7 days	6 years 7 days
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 21 days