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Earnings per Share (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Apr. 30, 2015
Apr. 30, 2014
Apr. 30, 2013
Net income allocated to common stockholders      
Net income $ 344.9 $ 565.2 $ 544.2
Net income allocated to participating securities 2.2 4.5 4.7
Net income allocated to common stockholders $ 342.7 $ 560.7 $ 539.5
Weighted-average common shares, basic and diluted      
Weighted-average common shares outstanding 103,038,271 103,504,121 107,881,519
Dilutive effect of stock options 5,283 14,346 23,256
Weighted-average common shares outstanding - assuming dilution 103,043,554 103,518,467 107,904,775
Computation of net income per common share      
Net income per common share $ 3.33 $ 5.42 $ 5.00
Net income per common share - assuming dilution $ 3.33 $ 5.42 $ 5.00