Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Jan. 31, 2017	Jul. 31, 2016	Jan. 31, 2016	Jan. 31, 2017	Jan. 31, 2016	May 01, 2016
Balance at April 30, 2016		$ 1,562		$ 1,562
Cumulative effect of change in accounting principle (Note 1)						$ 10
Net income	$ 182		$ 190	524	$ 545
Net other comprehensive income (loss)	(26)		$ (17)	(83)	$ (23)
Cash dividends				(273)
Acquisition of treasury stock				(561)
Stock-based compensation expense				10
Stock issued under compensation plans				11
Loss on issuance of treasury stock issued under compensation plans				(16)
Stock split				0
Balance at January 31, 2017	1,184			1,184
Additional Paid-in Capital [Member]
Balance at April 30, 2016		114		114
Stock-based compensation expense				10
Loss on issuance of treasury stock issued under compensation plans				(16)
Stock split		(34)		(34)
Balance at January 31, 2017	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				524
Cash dividends				(273)
Balance at January 31, 2017	4,326			4,326
AOCI Attributable to Parent [Member]
Balance at April 30, 2016		(350)		(350)
Net other comprehensive income (loss)				(83)
Balance at January 31, 2017	(433)			(433)
Treasury Stock, Common [Member]
Balance at April 30, 2016		(2,301)		(2,301)
Acquisition of treasury stock				(561)
Stock issued under compensation plans				11
Balance at January 31, 2017	(2,851)			(2,851)
Common stock, Class A, voting [Member] | Common Stock [Member]
Balance at April 30, 2016		13		13
Stock split				12
Balance at January 31, 2017	25			25
Common stock, Class B, nonvoting [Member] | Common Stock [Member]
Balance at April 30, 2016		$ 21		21
Stock split				22
Balance at January 31, 2017	$ 43			$ 43