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Pension and Other Postretirement Benefits (Funded Status of Plans) (Details) - USD ($)
$ in Millions
Apr. 30, 2020
Apr. 30, 2019
Apr. 30, 2018
Funded Status of Plans      
Assets $ 749 $ 754  
Pension Benefits [Member]      
Funded Status of Plans      
Assets 749 754 $ 780
Obligations (1,005) (908) (903)
Funded status (256) (154)  
Medical and Life Insurance Benefits [Member]      
Funded Status of Plans      
Assets 0 0 0
Obligations (51) (50) $ (50)
Funded status $ (51) $ (50)