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Assumptions Used in Estimating Fair Value of Stock Options Granted (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Share Based Compensation Arrangement Assumptions Used To Estimate Fair Values Of Share Options Granted [Line Items]
Expected term (in years)	3 years 8 months 12 days	3 years 8 months 12 days	3 years 7 months 28 days	3 years 9 months 18 days
Volatility	83.00%	97.00%	83.00%	101.00%
Annual dividend per share	$ 0.00	$ 0.00	$ 0.00	$ 0.00
Risk-free interest rate	1.54%	0.68%	0.99%	0.81%

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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