Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Basic and diluted earnings per share
Net income	$ 157.6	$ 133.1	$ 478.0	$ 408.7
Income allocated to participating securities	(0.1)	0	(0.2)	(0.1)
Net income available to common stockholders	$ 157.5	$ 133.1	$ 477.8	$ 408.6
Share data (in thousands):
Basic average common shares outstanding	213,459	212,891	213,301	214,976
Dilutive effect of stock-based awards	1,592	1,608	1,612	1,543
Diluted average common shares outstanding	215,051	214,499	214,913	216,519
Basic earnings per share	$ 0.74	$ 0.63	$ 2.24	$ 1.90
Diluted earnings per share	$ 0.73	$ 0.62	$ 2.22	$ 1.89