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Stock-Based Employee Compensation Plans - Stock Option pricing Weighted Average Assumptions (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Risk-free interest rate	1.00%	2.40%	2.50%
Expected volatility	24.60%	24.20%	25.00%
Dividend yield	2.40%	2.60%	2.90%
Expected average term (in years)	5 years 6 months	5 years 6 months	5 years 2 months 12 days

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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