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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, 2021
Apr. 30, 2020
Apr. 30, 2019
Pension Benefits [Member]      
Amounts recognized in OCI      
Net actuarial gain (loss) $ 69 $ (115) $ (41)
Amortization reclassified to earnings:      
Prior service cost (credit) 1 1 1
Net actuarial loss (gain) 27 20 34
Net amount recognized in OCI 97 (94) (6)
Medical and Life Insurance Benefits [Member]      
Amounts recognized in OCI      
Net actuarial gain (loss) 1 (2) 0
Amortization reclassified to earnings:      
Prior service cost (credit) (3) (3) (3)
Net actuarial loss (gain) 1 1 1
Net amount recognized in OCI $ (1) $ (4) $ (2)