Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Basic and diluted earnings per share
Net income available to common stockholders	$ 190	$ 182	$ 607	$ 524
Share data (in thousands):
Basic average common shares outstanding	480,361	480,650	480,193	486,105
Dilutive effect of stock-based awards	3,883	3,308	3,318	3,515
Diluted average common shares outstanding	484,244	483,958	483,511	489,620
Basic earnings per share (dollars per share)	$ 0.39	$ 0.38	$ 1.26	$ 1.08
Diluted earnings per share (dollars per share)	$ 0.39	$ 0.38	$ 1.25	$ 1.07