Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended
Jul. 31, 2012	Jul. 31, 2011
Basic and diluted earnings per share
Basic and diluted net income	$ 147.5	$ 118.1
Income allocated to participating securities	(0.1)
Net income available to common stockholders	$ 147.4	$ 118.1
Share data (in thousands):
Basic average common shares outstanding	213,168	217,242
Dilutive effect of stock-based awards	1,630	1,559
Diluted average common shares outstanding	214,798	218,801
Basic earnings per share	$ 0.69	$ 0.54
Diluted earnings per share	$ 0.69	$ 0.54

Net income available to common stockholders.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Aggregate of earnings distributed and earnings allocated to participating securities under the two-class method to the extent that participating securities may share in earnings as if all of the earnings for the period had been distributed.

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