Stock-based Compensation - Schedule of Assumptions Used to Estimate Fair Value of Each Employee and Non-employee Stock Award on Grant Date Using Black Scholes Option Pricing Model (Detail) - Employee and Nonemployee Stock Option [Member]	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend	0.00%	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility	79.00%	84.00%	85.00%
Expected term (in years)	5 years	5 years 6 months	6 years 3 months
Risk-free interest rate	1.45%	1.73%	1.07%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility	88.00%	95.00%	93.00%
Expected term (in years)	9 years 10 months 24 days	9 years 10 months 24 days	10 years
Risk-free interest rate	2.33%	2.15%	3.01%