Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended
	Jul. 31, 2014	Jul. 31, 2013
Basic and diluted earnings per share
Net income available to common stockholders	$ 150	$ 143
Share data (in thousands):
Basic average common shares outstanding	213,444	213,709
Dilutive effect of stock-based awards	1,575	1,611
Diluted average common shares outstanding	215,019	215,320
Basic earnings per share (dollars per share)	$ 0.70	$ 0.67
Diluted earnings per share (dollars per share)	$ 0.70	$ 0.66