Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Earnings Per Share [Abstract]
Net income available to common stockholders	$ 669	$ 1,067	$ 684
Share data (in thousands):
Basic average common shares outstanding (shares)	387,708	405,953	423,185
Dilutive effect of stock-based awards (shares)	2,753	2,607	2,980
Diluted average common shares outstanding (shares)	390,461	408,560	426,165
Basic earnings per share (dollars per share)	$ 1.72	$ 2.63	$ 1.62
Diluted earnings per share (dollars per share)	$ 1.71	$ 2.61	$ 1.60
Antidilutive common stock-based awards excluded from calculation of diluted earnings per share (shares)	1,716	905	723