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Pension and Other Postretirement Benefits (Changes in Funded Status of Benefit Plans Recognized in Other Comprehensive (Income) Loss) (Details) - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2020
Apr. 30, 2019
Apr. 30, 2018
Pension Benefits [Member]      
Amounts recognized in OCI      
Prior service credit (cost) $ 0 $ 0 $ (6)
Net actuarial gain (loss) (115) (41) 10
Amortization reclassified to earnings:      
Prior service cost (credit) 1 1 1
Net actuarial loss (gain) 20 34 21
Net amount recognized in OCI (94) (6) 26
Medical and Life Insurance Benefits [Member]      
Amounts recognized in OCI      
Prior service credit (cost) 0 0 0
Net actuarial gain (loss) (2) 0 1
Amortization reclassified to earnings:      
Prior service cost (credit) (3) (3) (3)
Net actuarial loss (gain) 1 1 1
Net amount recognized in OCI $ (4) $ (2) $ (1)