Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Oct. 31, 2016	Jul. 31, 2016	Oct. 31, 2015	Oct. 31, 2016	Oct. 31, 2015	May 01, 2016
Balance at April 30, 2016		$ 1,562		$ 1,562
Cumulative effect of change in accounting principle (Note 1)						$ 10
Net income	$ 197		$ 200	342	$ 356
Net other comprehensive income (loss)	(5)		$ (2)	(57)	$ (6)
Cash dividends				(134)
Acquisition of treasury stock				(442)
Stock-based compensation expense				7
Stock issued under compensation plans				8
Loss on issuance of treasury stock issued under compensation plans				(13)
Stock split				0
Balance at October 31, 2016	1,283			1,283
Additional Paid-in Capital [Member]
Balance at April 30, 2016		114		114
Stock-based compensation expense				7
Loss on issuance of treasury stock issued under compensation plans				(13)
Stock split		(34)		(34)
Balance at October 31, 2016	74			74
Retained Earnings [Member]
Balance at April 30, 2016		4,065		4,065
Cumulative effect of change in accounting principle (Note 1)						$ 10
Net income				342
Cash dividends				(134)
Balance at October 31, 2016	4,283			4,283
AOCI Attributable to Parent [Member]
Balance at April 30, 2016		(350)		(350)
Net other comprehensive income (loss)				(57)
Balance at October 31, 2016	(407)			(407)
Treasury Stock, Common [Member]
Balance at April 30, 2016		(2,301)		(2,301)
Acquisition of treasury stock				(442)
Stock issued under compensation plans				8
Balance at October 31, 2016	(2,735)			(2,735)
Common stock, Class A, voting [Member] | Common Stock [Member]
Balance at April 30, 2016		13		13
Stock split				12
Balance at October 31, 2016	25			25
Common stock, Class B, nonvoting [Member] | Common Stock [Member]
Balance at April 30, 2016		$ 21		21
Stock split				22
Balance at October 31, 2016	$ 43			$ 43