Pension and Other Postretirement Benefits (Change in Fair Value of Level 3 Assets) (Details) - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Apr. 30, 2021 |
Apr. 30, 2020 |
|
| Change in fair value of Level 3 Assets | ||
| Beginning balance | $ 749 | |
| Ending balance | 836 | $ 749 |
| Level 3 [Member] | ||
| Change in fair value of Level 3 Assets | ||
| Beginning balance | 2 | 3 |
| Return on assets held at end of year | 1 | |
| Sales and settlements | (1) | (1) |
| Ending balance | $ 2 | $ 2 |