Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Jan. 31, 2012	Jan. 31, 2011	Jan. 31, 2012	Jan. 31, 2011
Basic and diluted earnings per share
Net income	$ 133.1	$ 140.7	$ 408.7	$ 406.1
Income allocated to participating securities (restricted shares)		(0.1)	(0.1)	(0.4)
Net income available to common stockholders	$ 133.1	$ 140.6	$ 408.6	$ 405.7
Share data (in thousands):
Basic average common shares outstanding	141,928	145,061	143,317	145,787
Dilutive effect of stock options, SSARs, RSUs, and DSUs	1,072	979	1,029	883
Diluted average common shares outstanding	143,000	146,040	144,346	146,670
Basic earnings per share	$ 0.94	$ 0.97	$ 2.85	$ 2.78
Diluted earnings per share	$ 0.93	$ 0.96	$ 2.83	$ 2.77