Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2016	Oct. 31, 2015	Oct. 31, 2016	Oct. 31, 2015
Basic and diluted earnings per share
Net income available to common stockholders	$ 197	$ 200	$ 342	$ 356
Share data (in thousands):
Basic average common shares outstanding	389,050	408,110	390,994	411,116
Dilutive effect of stock-based awards	2,798	2,751	2,895	2,750
Diluted average common shares outstanding	391,848	410,861	393,889	413,866
Basic earnings per share (dollars per share)	$ 0.51	$ 0.49	$ 0.87	$ 0.87
Diluted earnings per share (dollars per share)	$ 0.50	$ 0.49	$ 0.87	$ 0.86