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Pension and Other Postretirement Benefits (Funded Status of Plans) (Details) - USD ($)
$ in Millions
Apr. 30, 2019
Apr. 30, 2018
Apr. 30, 2017
Funded Status of Plans      
Assets $ 754 $ 780  
Pension Benefits [Member]      
Funded Status of Plans      
Assets 754 780 $ 623
Obligations (908) (903) (893)
Funded status (154) (123)  
Medical and Life Insurance Benefits [Member]      
Funded Status of Plans      
Assets 0 0 0
Obligations (50) (50) $ (52)
Funded status $ (50) $ (50)