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Pension and Other Postretirement Benefits (Change in Fair Value of Level 3 Assets) (Details) - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2020
Apr. 30, 2019
Change in fair value of Level 3 Assets    
Beginning balance $ 754  
Ending balance 749 $ 754
Level 3 [Member]    
Change in fair value of Level 3 Assets    
Beginning balance 3 4
Sales and settlements (1) (1)
Ending balance $ 2 $ 3