Share-Based Awards and Options - Black-Scholes Valuation Model (Details) (USD $)	12 Months Ended
May 31, 2012 Y	May 31, 2011 Y	May 31, 2010 Y
2005 Plan Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option, weighted average grant-date fair values	$ 0	$ 11	$ 14
Risk-free interest rates	0.00%	1.74%	2.72%
Expected volatility	0.00%	31.96%	32.31%
Dividend yields	0.00%	0.21%	0.21%
Expected lives (in years)	0	5	5
Directors Plan Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option, weighted average grant-date fair values	$ 0	$ 12	$ 15
Risk-free interest rates	0.00%	1.31%	2.24%
Expected volatility	0.00%	31.96%	32.31%
Dividend yields	0.00%	0.21%	0.21%
Expected lives (in years)	0	5	5

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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