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Assumption of Estimated Fair Value of Options (Detail)	12 Months Ended
Assumption of Estimated Fair Value of Options (Detail)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Expected volatility	59.00%	72.00%	71.00%
Risk free interest rate	1.64%	2.15%	2.79%
Expected dividend yields
Expected lives	10 years	10 years	10 years

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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