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Weighted Average Assumptions Used to Determine Fair Value of Options Granted (Detail)	12 Months Ended		150 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	0.90%	1.90%	1.84%
Expected life of the options	5 years 9 months 22 days	5 years 1 month 17 days	5 years 2 months 9 days
Expected volatility of the underlying stock	116.00%	131.00%	119.00%
Expected dividend rate	0.00%	0.00%	0.00%

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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