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Assumptions To Value Stock Options (Detail)	12 Months Ended
Assumptions To Value Stock Options (Detail)	May 31, 2012	May 31, 2011	May 31, 2010
Non-Qualified Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	2.70%	2.80%	3.10%
Expected volatility	51.70%	53.80%	47.90%
Risk-free interest rate	1.90%	2.10%	2.90%
Expected life (years)	6 years	6 years	6 years
Market-Based Restricted Common Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	2.30%
Expected volatility	52.60%
Risk-free interest rate	1.80%

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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