Stock-Based Compensation (Details 1)	12 Months Ended
Apr. 30, 2011	Apr. 30, 2010	Apr. 30, 2009
Assumptions used for fair value estimation
Risk-free interest rate	2.10%	3.00%	3.50%
Expected volatility	23.70%	22.60%	18.10%
Expected dividend yield	1.90%	1.90%	1.80%
Expected life (years)	6	6	6

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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