Stockholders' Equity (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Oct. 31, 2018	Jul. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Oct. 31, 2018	Oct. 31, 2017	May 01, 2018
Beginning balance	$ 1,368	$ 1,316	$ 1,420	$ 1,370	$ 1,316	$ 1,370
Retirement of treasury stock				0
Net income	249	200	239	178	449	417
Net other comprehensive income (loss)	(1)	14	(15)	14	13	(1)
Cash dividends		(152)		(140)
Acquisition of treasury stock	(122)	(6)		(1)
Stock-based compensation expense	4	5	5	4
Stock issued under compensation plans	1	9	1	9
Loss on issuance of treasury stock issued under compensation plans	(2)	(13)	(3)	(14)
Cumulative effect of changes in accounting standards (Note 1)							$ (5)
Ending balance	1,497	1,368	1,647	1,420	1,497	1,647
Additional Paid-in Capital [Member]
Beginning balance	2	4	47	65	4	65
Retirement of treasury stock				8
Stock-based compensation expense	4	5	5	4
Loss on issuance of treasury stock issued under compensation plans	(2)	(7)	(3)	(14)
Ending balance	4	2	49	47	4	49
Retained Earnings [Member]
Beginning balance	1,767	1,730	1,824	4,470	1,730	4,470
Retirement of treasury stock				2,684
Net income	249	200	239	178
Cash dividends		(152)		(140)
Loss on issuance of treasury stock issued under compensation plans		(6)
Cumulative effect of changes in accounting standards (Note 1)							$ (5)
Ending balance	2,016	1,767	2,063	1,824	2,016	2,063
AOCI Attributable to Parent [Member]
Beginning balance	(364)	(378)	(376)	(390)	(378)	(390)
Net other comprehensive income (loss)	(1)	14	(15)	14
Ending balance	(365)	(364)	(391)	(376)	(365)	(391)
Treasury Stock, Common [Member]
Beginning balance	(109)	(112)	(133)	(2,843)	(112)	(2,843)
Retirement of treasury stock				(2,702)
Acquisition of treasury stock	(122)	(6)		(1)
Stock issued under compensation plans	1	9	1	9
Ending balance	(230)	(109)	(132)	(133)	(230)	(132)
Common stock, Class A, voting [Member] \| Common Stock [Member]
Beginning balance	25	25	25	25	25	25
Ending balance	25	25	25	25	25	25
Common stock, Class B, nonvoting [Member] \| Common Stock [Member]
Beginning balance	47	47	33	43	47	43
Retirement of treasury stock				10
Ending balance	$ 47	$ 47	$ 33	$ 33	$ 47	$ 33

Amount of increase to additional paid-in capital (APIC) from recognition of equity-based compensation.

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