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Summary of Weighted-Average Assumptions Related to Grants (Detail)	12 Months Ended
	May 31, 2013	May 31, 2012	May 31, 2011
Schedule Of Weighted Average Assumptions [Line Items]
Risk-free interest rate	1.10%	2.50%	2.10%
Expected life of option	7 years 6 months	7 years 6 months	7 years 6 months
Expected dividend yield	3.30%	3.80%	4.10%
Expected volatility rate	28.20%	29.50%	29.60%

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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