Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Jan. 31, 2014	Jan. 31, 2013	Jan. 31, 2014	Jan. 31, 2013
Basic and diluted earnings per share
Net income	$ 177	$ 157	$ 526	$ 478
Net income available to common stockholders	$ 177	$ 157	$ 526	$ 478
Share data (in thousands):
Basic average common shares outstanding	213,151	213,459	213,493	213,301
Dilutive effect of stock-based awards	1,641	1,592	1,623	1,612
Diluted average common shares outstanding	214,792	215,051	215,116	214,913
Basic earnings per share	$ 0.83	$ 0.74	$ 2.46	$ 2.24
Diluted earnings per share	$ 0.82	$ 0.73	$ 2.45	$ 2.22