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Range of Assumptions (Detail)	12 Months Ended
Range of Assumptions (Detail)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Stock Options [Line Items]
Expected dividend rate	0.00%	0.00%	0.00%
Expected stock price volatility	45.00%	45.00%	45.00%
Risk-free interest rate, minimum	1.00%	1.70%	1.90%
Risk-free interest rate, maximum	1.60%	3.00%	3.20%
Expected term of options		7 years 6 months
Minimum
Schedule Of Stock Options [Line Items]
Expected term of options	6 years 6 months		3 years 6 months
Maximum
Schedule Of Stock Options [Line Items]
Expected term of options	7 years 6 months		7 years 6 months

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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