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Fair Values of Employee Stock Options (Detail)	3 Months Ended		6 Months Ended
Fair Values of Employee Stock Options (Detail)	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Share Based Compensation Arrangement Assumptions Used To Estimate Fair Values Of Share Options Granted [Line Items]
Risk-free interest rate	1.20%	1.20%	1.10%	1.20%
Expected life	6 years	6 years	6 years	6 years
Volatility	79.00%	70.00%	79.00%	70.00%
Dividend yield

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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