Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Jan. 31, 2015	Jan. 31, 2014	Jan. 31, 2015	Jan. 31, 2014
Basic and diluted earnings per share
Net income available to common stockholders	$ 186	$ 177	$ 543	$ 526
Share data (in thousands):
Basic average common shares outstanding	211,126	213,151	212,189	213,493
Dilutive effect of stock-based awards	1,480	1,641	1,512	1,623
Diluted average common shares outstanding	212,606	214,792	213,701	215,116
Basic earnings per share (dollars per share)	$ 0.88us-gaap_EarningsPerShareBasic	$ 0.83us-gaap_EarningsPerShareBasic	$ 2.56us-gaap_EarningsPerShareBasic	$ 2.46us-gaap_EarningsPerShareBasic
Diluted earnings per share (dollars per share)	$ 0.87us-gaap_EarningsPerShareDiluted	$ 0.82us-gaap_EarningsPerShareDiluted	$ 2.54us-gaap_EarningsPerShareDiluted	$ 2.45us-gaap_EarningsPerShareDiluted

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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