Fair Value of Employee Stock Options Assumptions (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Risk free interest rate, minimum	1.83%	1.18%	1.61%
Risk free interest rate, maximum	2.13%	1.80%	1.86%
Volatility, minimum	83.32%	77.57%	69.76%
Volatility, maximum	90.39%	82.75%	71.84%
Minimum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Expected lives	5 years 4 months 28 days	5 years 1 month 16 days	5 years 9 months 14 days
Maximum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Expected lives	5 years 9 months 10 days	5 years 9 months 25 days	5 years 10 months 10 days