Assumptions Used in Estimating Fair Value of Stock Options Granted (Detail) - 2002 Stock Incentive Plan	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	5 years 3 months 18 days	5 years 10 months 24 days	5 years 6 months	5 years 9 months 18 days
Volatility	80.50%	86.60%	78.70%	85.50%
Risk-free interest rate	1.76%	1.30%	1.95%	1.32%