Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2015	Oct. 31, 2014	Oct. 31, 2015	Oct. 31, 2014
Basic and diluted earnings per share
Net income available to common stockholders	$ 200	$ 208	$ 356	$ 358
Share data (in thousands):
Basic average common shares outstanding	204,055	212,087	205,558	212,674
Dilutive effect of stock-based awards	1,376	1,482	1,375	1,528
Diluted average common shares outstanding	205,431	213,569	206,933	214,202
Basic earnings per share (dollars per share)	$ 0.98	$ 0.98	$ 1.73	$ 1.68
Diluted earnings per share (dollars per share)	$ 0.97	$ 0.97	$ 1.72	$ 1.67