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Assumptions Used in Estimating Fair Value of Stock Options Granted (Detail) - 2002 Stock Incentive Plan
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (years) 5 years 6 months 3 days 5 years 11 months 15 days 5 years 9 months 18 days
Volatility 79.00% 86.00% 90.00%
Risk-free interest rate 1.99% 1.39% 1.64%