Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Jul. 31, 2018	Jul. 31, 2017
Basic and diluted earnings per share
Net income available to common stockholders	$ 200	$ 178
Share data (in thousands):
Basic average common shares outstanding	480,964	480,048
Dilutive effect of stock-based awards	3,477	2,936
Diluted average common shares outstanding	484,441	482,984
Basic earnings per share (dollars per share)	$ 0.42	$ 0.37
Diluted earnings per share (dollars per share)	$ 0.41	$ 0.37