Quarterly report pursuant to Section 13 or 15(d)

Assumptions Used in Estimating Fair Value of Stock Options Granted (Detail)

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Assumptions Used in Estimating Fair Value of Stock Options Granted (Detail) - 2002 Stock Incentive Plan
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Expected term 5 years 10 months 24 days 5 years 9 months 18 days
Volatility 81.40% 79.40%
Risk-free interest rate 2.46% 1.98%