Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016
Basic and diluted earnings per share
Net income available to common stockholders	$ 239	$ 197	$ 417	$ 342
Share data (in thousands):
Basic average common shares outstanding	384,120	389,050	384,076	390,994
Dilutive effect of stock-based awards	2,507	2,798	2,428	2,895
Diluted average common shares outstanding	386,627	391,848	386,504	393,889
Basic earnings per share (dollars per share)	$ 0.62	$ 0.51	$ 1.08	$ 0.87
Diluted earnings per share (dollars per share)	$ 0.62	$ 0.50	$ 1.08	$ 0.87