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Fair Value Assumptions on Stock Option (Detail) (USD $)	9 Months Ended
Fair Value Assumptions on Stock Option (Detail) (USD $)	Sep. 30, 2012	Sep. 30, 2011
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Weighted-average fair value of grant	$ 9.73	$ 20.12
Risk-free interest rate	0.54%	1.71%
Expected volatility	71.29%	71.31%
Expected life of options in years	3 years 7 months 24 days	3 years 7 months 13 days
Expected 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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