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Weighted-Average Assumptions (Detail) (Stock Options)	12 Months Ended
Weighted-Average Assumptions (Detail) (Stock Options)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock Options
Share Based Payment Award Stock Options Valuation Assumptions [Line Items]
Risk-free interest rate	0.77%	2.26%	2.12%
Expected volatility	59.00%	41.00%	54.00%
Expected life of the option in years	4 years 7 months 6 days	4 years 7 months 6 days	4 years 7 months 21 days
Expected dividend yield	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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