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Stock-Based Compensation (Stock Options and SSARs Fair Value Assumptions) (Details)	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Assumptions used for fair value estimation
Risk-free interest rate	0.90%	1.90%	2.10%
Expected volatility	22.90%	23.60%	23.70%
Expected dividend yield	1.90%	1.90%	1.90%
Expected term (years)	6 years 6 months	6 years	6 years

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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