Assumptions for Measuring Fair Values of Stock Options Granted (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.60%	0.90%	1.40%
Expected lives	4 years 9 months 18 days	5 years 2 months 12 days	5 years 3 months 18 days
Expected volatility	79.70%	78.20%	82.50%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.10%	2.40%	3.80%
Expected lives	5 years 3 months 18 days	8 years 10 months 24 days	8 years
Expected volatility	108.60%	81.60%	90.30%