Stock-Based Compensation (Textual) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
The total intrinsic value of options and SSARs exercised	$ 73	$ 28	$ 47
Stock options and SSARs award vesting period (years)	3 years
Stock options and SSARs award expiration period (years)	7 years
Grant-date fair value per award (dollars per share)	$ 6.79	$ 5.73	$ 7.62
Stock-based incentive awards on a pre-tax	$ 19	$ 14	$ 15
Compensation expense partially offset by deferred income tax benefits	6	5	6
Total unrecognized compensation cost related to non-vested stock-based compensation	$ 7
Unrecognized compensation cost, weighted-average period of recognition (years)	1 year 3 months 18 days
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding under Restricted stock units (shares)	670
Weighted-average remaining restriction period (years)	9 months 18 days
Total fair value of RSUs, restricted stock, and DSUs vested	$ 6	$ 8	$ 10
Omnibus Compensation Plan 2004 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized under 2013 Omnibus Compensation Plan (shares)	20,750
Shares issued under 2013 Omnibus Compensation Plan (shares)	14,791

Share based compensation arrangement by award options and stock settled appreciation rights award expiration period.

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- Definition

Share based compensation arrangement by award options and stock settled appreciation rights award vesting period.

+ References

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- Definition

Share based compensation arrangement by award options and stock settled appreciation rights exercises in period intrinsic value.

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- Definition

Share based compensation arrangement by award options and stock settled appreciation rights grants in period weighted average grant date fair value.

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- Definition

Share based compensation arrangement by other than options weighted average remaining restriction period.

+ References

No definition available.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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-Paragraph 2
-Subparagraph (a)(3)
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- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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