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Share-Based Compensation Plans - Assumptions Used to Determine Weighted-Average Fair Value of Options Granted (Detail)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Assumptions used to determine weighted-average fair value of options granted
Expected dividend yield	4.40%	4.30%
Risk-free interest rate	1.30%	2.60%
Expected volatility	25.30%	23.20%
Expected life (years)	7 years	7 years

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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