Pension and Other Postretirement Benefits (Fair Value of Pension Plan Assets and Asset Allocations) (Details) - USD ($) $ in Millions		Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		$ 749	$ 754
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		80	108
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	0
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		2	3	$ 4
Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		82	111
Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		80	79
Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	0
Equity Securities [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	0
Equity Securities [Member] | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		80	79
Cash And Temporary Investments [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	29
Cash And Temporary Investments [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	0
Cash And Temporary Investments [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	0
Cash And Temporary Investments [Member] | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets		0	29
Limited Partnership Interests [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0	0
Limited Partnership Interests [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	0	0
Limited Partnership Interests [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	2	3
Limited Partnership Interests [Member] | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[1]	2	3
Limited Partnership Interests [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[2]	32	27
Equity Funds [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	193	157
Fixed Income Funds [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	370	370
Real Estate funds [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	68	66
Short-term Investments [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, Plan Assets	[3]	$ 4	$ 23

[1] 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 and the inherent lack of liquidity. This limited partnership has a term expiring in September 2020, although this period may be extended.

[2] 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 extended.

[3] 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. Generally, for 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.