Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Jul. 31, 2017	Jul. 31, 2016
Basic and diluted earnings per share
Net income available to common stockholders	$ 178	$ 144
Share data (in thousands):
Basic average common shares outstanding	384,038	393,018
Dilutive effect of stock-based awards	2,349	2,991
Diluted average common shares outstanding	386,387	396,009
Basic earnings per share (dollars per share)	$ 0.46	$ 0.37
Diluted earnings per share (dollars per share)	$ 0.46	$ 0.36