Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
Jul. 31, 2016	Jul. 31, 2015
Basic and diluted earnings per share
Net income available to common stockholders	$ 144	$ 156
Share data (in thousands):
Basic average common shares outstanding	393,018	414,526
Dilutive effect of stock-based awards	2,991	2,750
Diluted average common shares outstanding	396,009	417,276
Basic earnings per share (dollars per share)	$ 0.37	$ 0.38
Diluted earnings per share (dollars per share)	$ 0.36	$ 0.37

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 average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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