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Assumptions Used in Estimating Fair Value Of Stock Options Granted (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share based Compensation Arrangement Assumptions Used to Estimate Fair Values of Share Options Granted [Line Items]
Expected term (years)	3 years 8 months 12 days	4 years 1 month 24 days	4 years 6 months 15 days
Volatility	97.00%	105.00%	91.00%
Annual dividend per share	$ 0.00	$ 0.00	$ 0.00
Risk-free interest rate	0.77%	1.53%	1.94%

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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