Earnings per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Jul. 31, 2013	Jul. 31, 2012
Net income (loss) allocated to common stockholders
Net income	$ 126.6	$ 110.9
Net income allocated to participating securities	1.1	1.0
Net income allocated to common stockholders	$ 125.5	$ 109.9
Weighted-average common shares, basic and diluted
Weighted-average common shares outstanding	105,077,523	109,418,592
Dilutive effect of stock options	16,755	29,536
Weighted-average common shares outstanding - assuming dilution	105,094,278	109,448,128
Computation of net income per common share:
Net income per common share	$ 1.19	$ 1.00
Net income per common share - assuming dilution	$ 1.19	$ 1.00