Earnings per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2014	Jan. 31, 2013
Net income (loss) allocated to common stockholders
Net income	$ 166.7	$ 154.2	$ 446.7	$ 413.9
Net income allocated to participating securities	1.2	1.4	3.5	3.6
Net income allocated to common stockholders	$ 165.5	$ 152.8	$ 443.2	$ 410.3
Weighted-average common shares, basic and diluted
Weighted-average common shares outstanding	103,980,819	107,528,722	104,456,496	108,405,604
Dilutive effect of stock options	13,623	19,655	15,210	25,263
Weighted-average common shares outstanding - assuming dilution	103,994,442	107,548,377	104,471,706	108,430,867
Computation of net income per common share:
Net income per common share	$ 1.59	$ 1.42	$ 4.24	$ 3.78
Net income per common share - assuming dilution	$ 1.59	$ 1.42	$ 4.24	$ 3.78