Pension and Other Postretirement Benefits (Fair Value of Pension Plan Assets and Asset Allocations) (Details) - USD ($) $ in Millions		Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		$ 780	$ 623
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		4	4	$ 5
Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	226	206
Fixed Income Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	362	229
Real Estate funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	66	63
Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	5	7
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[2]	1	8
Limited Partnership Interests [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	27	28
Included in Fair Value Hierarchy [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		93	82
Included in Fair Value Hierarchy [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		89	78
Included in Fair Value Hierarchy [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	0
Included in Fair Value Hierarchy [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		4	4
Included in Fair Value Hierarchy [Member] | Limited Partnership Interests [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[4]	4	4
Included in Fair Value Hierarchy [Member] | Limited Partnership Interests [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[4]	0	0
Included in Fair Value Hierarchy [Member] | Limited Partnership Interests [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[4]	0	0
Included in Fair Value Hierarchy [Member] | Limited Partnership Interests [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[4]	4	4
Included in Fair Value Hierarchy [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		89	78
Included in Fair Value Hierarchy [Member] | Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		89	78
Included in Fair Value Hierarchy [Member] | Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	0
Included in Fair Value Hierarchy [Member] | Equity Securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		$ 0	$ 0

[1] Commingled trust fund valuations are based on the NAV of the funds as determined by the fund administrators and reviewed by us. NAV represents the underlying assets owned by the fund, minus liabilities and divided by the number of shares or units outstanding. For primarily all commingled trust funds other than real estate, redemptions are permitted daily with no notice period. The real estate fund is redeemable quarterly with 110 days notice.

[2] Hedge fund valuations are based primarily on the NAV of the funds as determined by fund administrators and reviewed by us. During our review, we determine whether it is necessary to adjust a valuation for inherent liquidity and redemption issues that may exist within a fund’s underlying assets or fund unit values.

[3] These limited partnership interests were initially valued at cost and have been adjusted using NAV per audited financial statements. Investments are generally not eligible for immediate redemption and have original terms averaging 10 to 13 years, although those periods may be subsequently extended.

[4] This limited partnership interest was initially valued at cost and has been adjusted to fair value as determined in good faith by management of the partnership using various factors, and does not meet the requirements for reporting at the net asset value (NAV). The valuation requires significant judgment due to the absence of quoted market prices, the inherent lack of liquidity, and the long-term nature of the investment.