Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2014	Oct. 31, 2013
Basic and diluted earnings per share
Net income available to common stockholders	$ 208	$ 206	$ 358	$ 349
Share data (in thousands):
Basic average common shares outstanding	212,087	213,587	212,674	213,634
Dilutive effect of stock-based awards	1,482	1,617	1,528	1,614
Diluted average common shares outstanding	213,569	215,204	214,202	215,248
Basic earnings per share (dollars per share)	$ 0.98	$ 0.97	$ 1.68	$ 1.63
Diluted earnings per share (dollars per share)	$ 0.97	$ 0.96	$ 1.67	$ 1.62