Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2017	Jan. 31, 2016
Basic and diluted earnings per share
Net income available to common stockholders	$ 182	$ 190	$ 524	$ 545
Share data (in thousands):
Basic average common shares outstanding	384,520	402,365	388,884	408,483
Dilutive effect of stock-based awards	2,646	2,416	2,812	2,668
Diluted average common shares outstanding	387,166	404,781	391,696	411,151
Basic earnings per share (dollars per share)	$ 0.47	$ 0.47	$ 1.35	$ 1.33
Diluted earnings per share (dollars per share)	$ 0.47	$ 0.47	$ 1.34	$ 1.33