Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2011	Oct. 31, 2010	Oct. 31, 2011	Oct. 31, 2010
Basic and diluted earnings per share
Net income	$ 157.6	$ 154.0	$ 275.6	$ 265.4
Income allocated to participating securities (restricted shares)	(0.1)	(0.2)	(0.1)	(0.3)
Net income available to common stockholders	$ 157.5	$ 153.8	$ 275.5	$ 265.1
Share data (in thousands):
Basic average common shares outstanding	143,209	145,649	143,912	146,113
Dilutive effect of stock options, SSARs, RSUs, and DSUs	975	855	1,007	835
Diluted average common shares outstanding	144,184	146,504	144,919	146,948
Basic earnings per share	$ 1.10	$ 1.06	$ 1.91	$ 1.81
Diluted earnings per share	$ 1.09	$ 1.05	$ 1.90	$ 1.80