Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Jan. 31, 2019	Jan. 31, 2018	May 01, 2018
Beginning balance	$ 1,497	$ 1,368	$ 1,316	$ 1,647	$ 1,420	$ 1,370	$ 1,316	$ 1,370
Retirement of treasury stock						0
Net income	227	249	200	190	239	178	676	607
Net other comprehensive income (loss)	5	(1)	14	9	(15)	14	18	8
Declaration of cash dividends	(158)		(152)	(633)		(140)
Acquisition of treasury stock	(78)	(122)	(6)			(1)
Stock-based compensation expense	3	4	5	5	5	4
Stock issued under compensation plans	1	1	9	16	1	9
Loss on issuance of treasury stock issued under compensation plans	(4)	(2)	(13)	(33)	(3)	(14)
Cumulative effect of changes in accounting standards (Note 1)									$ (5)
Stock split				0
Ending balance	1,493	1,497	1,368	1,201	1,647	1,420	1,493	1,201
Additional Paid-in Capital [Member]
Beginning balance	4	2	4	49	47	65	4	65
Retirement of treasury stock						8
Stock-based compensation expense	3	4	5	5	5	4
Loss on issuance of treasury stock issued under compensation plans	4	(2)	(7)	(33)	(3)	(14)
Stock split				(14)
Ending balance	3	4	2	7	49	47	3	7
Retained Earnings [Member]
Beginning balance	2,016	1,767	1,730	2,063	1,824	4,470	1,730	4,470
Retirement of treasury stock						2,684
Net income	227	249	200	190	239	178
Declaration of cash dividends	(158)		(152)	(633)		(140)
Loss on issuance of treasury stock issued under compensation plans			(6)
Cumulative effect of changes in accounting standards (Note 1)									$ (5)
Ending balance	2,085	2,016	1,767	1,620	2,063	1,824	2,085	1,620
AOCI Attributable to Parent [Member]
Beginning balance	(365)	(364)	(378)	(391)	(376)	(390)	(378)	(390)
Net other comprehensive income (loss)	5	(1)	14	9	(15)	14
Ending balance	(360)	(365)	(364)	(382)	(391)	(376)	(360)	(382)
Treasury Stock, Common [Member]
Beginning balance	(230)	(109)	(112)	(132)	(133)	(2,843)	(112)	(2,843)
Retirement of treasury stock						(2,702)
Acquisition of treasury stock	78	(122)	(6)			(1)
Stock issued under compensation plans	1	1	9	16	1	9
Ending balance	(307)	(230)	(109)	(116)	(132)	(133)	(307)	(116)
Common stock, Class A, voting [Member] \| Common Stock [Member]
Beginning balance	25	25	25	25	25	25	25	25
Ending balance	25	25	25	25	25	25	25	25
Common stock, Class B, nonvoting [Member] \| Common Stock [Member]
Beginning balance	47	47	47	33	33	43	47	43
Retirement of treasury stock						10
Stock split								14
Ending balance	$ 47	$ 47	$ 47	$ 47	$ 33	$ 33	$ 47	$ 47

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