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Summary Of Assumptions For Options Granted (Detail)	12 Months Ended
Summary Of Assumptions For Options Granted (Detail)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share based Compensation Arrangement Assumptions Used to Estimate Fair Values of Share Options Granted [Line Items]
Volatility factor	39.40%	42.30%	40.30%
Dividend yield	1.00%	1.00%	1.30%
Expected term (in years)	5 years 6 months 18 days	4 years 7 months 28 days	4 years 8 months 27 days
Risk-free rate	1.30%	2.00%	2.50%

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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