Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended
	Jul. 31, 2017	Jul. 31, 2016
Balance at April 30, 2017	$ 1,370
Retirement of treasury stock	0
Net income	178	$ 144
Net other comprehensive income (loss)	14	$ (52)
Cash dividends	(140)
Acquisition of treasury stock	(1)
Stock-based compensation expense	4
Stock issued under compensation plans	9
Loss on issuance of treasury stock issued under compensation plans	(14)
Balance at July 31, 2017	1,420
Additional Paid-in Capital [Member]
Balance at April 30, 2017	65
Retirement of treasury stock	(8)
Stock-based compensation expense	4
Loss on issuance of treasury stock issued under compensation plans	(14)
Balance at July 31, 2017	47
Retained Earnings [Member]
Balance at April 30, 2017	4,470
Retirement of treasury stock	(2,684)
Net income	178
Cash dividends	(140)
Balance at July 31, 2017	1,824
AOCI Attributable to Parent [Member]
Balance at April 30, 2017	(390)
Net other comprehensive income (loss)	14
Balance at July 31, 2017	(376)
Treasury Stock, Common [Member]
Balance at April 30, 2017	(2,843)
Retirement of treasury stock	2,702
Acquisition of treasury stock	(1)
Stock issued under compensation plans	9
Balance at July 31, 2017	(133)
Common stock, Class A, voting [Member] | Common Stock [Member]
Balance at April 30, 2017	25
Balance at July 31, 2017	25
Common stock, Class B, nonvoting [Member] | Common Stock [Member]
Balance at April 30, 2017	43
Retirement of treasury stock	(10)
Balance at July 31, 2017	$ 33