Accounting Policies (Basis and Diluted Earnings Per Share) (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Basic and diluted earnings per share
Basic and diluted net income	$ 591	$ 513	$ 572
Income allocated to participating securities	0	0	(1)
Net income available to common stockholders	$ 591	$ 513	$ 571
Share data (in thousands):
Basic average common shares outstanding (shares)	213,369	214,529	218,405
Dilutive effect of stock-based awards (shares)	1,617	1,554	1,365
Diluted average common shares outstanding (shares)	214,986	216,083	219,770
Basic earnings per share (dollars per share)	$ 2.77	$ 2.39	$ 2.61
Diluted earnings per share (dollars per share)	$ 2.75	$ 2.37	$ 2.60