Pension and Other Postretirement Benefits (Fair Value of Pension Plan Assets and Asset Allocations) (Details) - USD ($)
$ in Millions
Apr. 30, 2021
Apr. 30, 2020
Apr. 30, 2019
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets $ 836 $ 749  
Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets 103 80  
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 2 $ 3
Fair Value, Inputs, Level 1, 2 and 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets 105 82  
Equity Securities [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets 103 80  
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 103 80  
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 2  
Limited Partnership Interests [Member] | Fair Value, Inputs, Level 1, 2 and 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets [1] 2 2  
Limited Partnership Interests [Member] | Fair Value Measured at Net Asset Value Per Share [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets [2] 35 32  
Equity Funds [Member] | Fair Value Measured at Net Asset Value Per Share [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets [3] 266 193  
Fixed Income Funds [Member] | Fair Value Measured at Net Asset Value Per Share [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets [3] 357 370  
Real Estate funds [Member] | Fair Value Measured at Net Asset Value Per Share [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets [3] 65 68  
Short-term Investments [Member] | Fair Value Measured at Net Asset Value Per Share [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total, Plan Assets [3] $ 8 $ 4  
[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 2021, 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.