Pension and Other Postretirement Benefits (Change in Fair Value of Level 3 Assets) (Details) - USD ($) $ in Millions	12 Months Ended
Apr. 30, 2017	Apr. 30, 2016
Change in fair value of Level 3 Assets
Beginning balance	$ 594
Ending balance	623	$ 594
Level 3 [Member]
Change in fair value of Level 3 Assets
Beginning balance	29	26
Return on assets held at end of year	1	1
Purchases and settlements	5	5
Sales and settlements	(3)	(3)
Ending balance	$ 32	$ 29

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6749337

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6520858

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant