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Weighted Average Assumptions Used to Estimate Fair Values (Detail)	3 Months Ended
	Aug. 31, 2012	Aug. 31, 2011
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Method Used [Line Items]
Dividend yield	1.50%	1.40%
Expected volatility	35.00%	29.40%
Weighted average expected life (in years)	5 years 3 months 18 days	5 years
Risk-free interest rate	0.60%	1.50%

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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