Earnings per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2014	Oct. 31, 2013
Net income (loss) allocated to common stockholders
Net income	$ 158.3	$ 153.4	$ 274.3	$ 280.0
Net income allocated to participating securities	1.1	1.2	1.9	2.3
Net income allocated to common stockholders	$ 157.2	$ 152.2	$ 272.4	$ 277.7
Weighted-average common shares, basic and diluted
Weighted-average common shares outstanding	101,123,147	104,311,146	101,075,885	104,694,335
Dilutive effect of stock options	3,250	15,251	5,860	16,003
Weighted-average common shares outstanding - assuming dilution	101,126,397	104,326,397	101,081,745	104,710,338
Computation of net income per common share:
Net income per common share	$ 1.55	$ 1.46	$ 2.69	$ 2.65
Net income per common share - assuming dilution	$ 1.55	$ 1.46	$ 2.69	$ 2.65