Summary of Weighted-Average Assumptions Related to Grants (Detail)	12 Months Ended
May 31, 2012 Year	May 31, 2011 Year	May 31, 2010 Year
Schedule of Weighted Average Assumptions for Fair Values of Stock Options[Line Items]
Risk-free interest rate	2.50%	2.10%	3.20%
Expected life of option	7.5	7.5	7.5
Expected dividend yield	3.80%	4.10%	4.30%
Expected volatility rate	29.50%	29.60%	30.40%

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

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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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