Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Jul. 31, 2015	Jul. 31, 2014
Basic and diluted earnings per share
Net income available to common stockholders	$ 156	$ 150
Share data (in thousands):
Basic average common shares outstanding	207,263	213,444
Dilutive effect of stock-based awards	1,375	1,575
Diluted average common shares outstanding	208,638	215,019
Basic earnings per share (dollars per share)	$ 0.75	$ 0.70
Diluted earnings per share (dollars per share)	$ 0.75	$ 0.70