Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012	Oct. 31, 2013	Oct. 31, 2012
Basic and diluted earnings per share
Net income	$ 206	$ 173	$ 349	$ 320
Net income available to common stockholders	$ 206	$ 173	$ 349	$ 320
Share data (in thousands):
Basic average common shares outstanding	213,587	213,276	213,634	213,220
Dilutive effect of stock-based awards	1,617	1,615	1,614	1,623
Diluted average common shares outstanding	215,204	214,891	215,248	214,843
Basic earnings per share	$ 0.97	$ 0.81	$ 1.63	$ 1.50
Diluted earnings per share	$ 0.96	$ 0.80	$ 1.62	$ 1.49