Preferred Stock Warrant Liability - Summary of Assumptions and Inputs Used in Determining Fair Value of Preferred Stock Warrant Liability Valued Using Black-Scholes Option-Pricing Model (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	2.40%	2.17%	3.20%
Expected term (in years)	8 years 2 months 12 days	8 years 8 months 12 days	9 years 8 months 12 days
Expected volatility	91.20%	84.00%	86.00%
Expected dividend yield	0.00%	0.00%	0.00%
Fair value of Series A-2 convertible preferred stock	$ 0	$ 1,582,000	$ 164,000		$ 0
Series A-2 Convertible Preferred Stock [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value of Series A-2 convertible preferred stock	$ 17,260	$ 17,180	$ 2,070	$ 156,000