Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Jan. 31, 2016	Jan. 31, 2015	Jan. 31, 2016	Jan. 31, 2015
Basic and diluted earnings per share
Net income available to common stockholders	$ 190	$ 186	$ 545	$ 543
Share data (in thousands):
Basic average common shares outstanding	201,182	211,126	204,242	212,189
Dilutive effect of stock-based awards	1,208	1,480	1,334	1,512
Diluted average common shares outstanding	202,390	212,606	205,576	213,701
Basic earnings per share (dollars per share)	$ 0.94	$ 0.88	$ 2.67	$ 2.56
Diluted earnings per share (dollars per share)	$ 0.94	$ 0.87	$ 2.65	$ 2.54