Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016
Balance at April 30, 2017			$ 1,370
Retirement of treasury stock			0
Net income	$ 239	$ 197	417	$ 342
Net other comprehensive income (loss)	(15)	$ (5)	(1)	$ (57)
Cash dividends			(140)
Acquisition of treasury stock			(1)
Stock-based compensation expense			9
Stock issued under compensation plans			10
Loss on issuance of treasury stock issued under compensation plans			(17)
Balance at October 31, 2017	1,647		1,647
Additional Paid-in Capital [Member]
Balance at April 30, 2017			65
Retirement of treasury stock			(8)
Stock-based compensation expense			9
Loss on issuance of treasury stock issued under compensation plans			(17)
Balance at October 31, 2017	49		49
Retained Earnings [Member]
Balance at April 30, 2017			4,470
Retirement of treasury stock			(2,684)
Net income			417
Cash dividends			(140)
Balance at October 31, 2017	2,063		2,063
AOCI Attributable to Parent [Member]
Balance at April 30, 2017			(390)
Net other comprehensive income (loss)			(1)
Balance at October 31, 2017	(391)		(391)
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			10
Balance at October 31, 2017	(132)		(132)
Common stock, Class A, voting [Member] | Common Stock [Member]
Balance at April 30, 2017			25
Balance at October 31, 2017	25		25
Common stock, Class B, nonvoting [Member] | Common Stock [Member]
Balance at April 30, 2017			43
Retirement of treasury stock			(10)
Balance at October 31, 2017	$ 33		$ 33