Assumptions Used in Estimating Fair Value of Stock Options Granted (Detail) - 2002 Stock Incentive Plan	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	5 years 4 months 24 days	5 years 7 months 6 days	5 years 7 months 6 days	5 years 8 months 12 days
Volatility	77.60%	83.80%	78.50%	84.80%
Risk-free interest rate	1.98%	1.32%	1.98%	1.34%