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Stockholders' Equity - Fair Value of Share-Based Payments was Estimated on Date of Grant Using Black-Scholes Option Pricing Model Based on Weighted Average Assumptions (Detail)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
ESPP [Member]      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Risk-free interest rate 0.50% 0.30% 0.20%
Volatility 74.00% 75.30% 86.00%
Expected term in years 6 months 6 months 22 days 1 year 3 months
Expected dividend yield 0.00% 0.00% 0.00%
Options [Member]      
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]      
Risk-free interest rate 1.90% 1.70% 1.90%
Volatility 74.00% 79.40% 77.10%
Expected term in years 6 years 5 months 9 days 6 years 4 months 17 days 6 years 3 months 18 days
Expected dividend yield 0.00% 0.00% 0.00%