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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, 2019
Apr. 30, 2018
Apr. 30, 2017
Pension Benefits [Member]      
Amounts recognized in OCI      
Prior service credit (cost) $ 0 $ (6) $ (1)
Net actuarial gain (loss) (41) 10 24
Amortization reclassified to earnings:      
Prior service cost (credit) 1 1 1
Net actuarial loss (gain) 34 21 26
Net amount recognized in OCI (6) 26 50
Medical and Life Insurance Benefits [Member]      
Amounts recognized in OCI      
Prior service credit (cost) 0 0 4
Net actuarial gain (loss) 0 1 0
Amortization reclassified to earnings:      
Prior service cost (credit) (3) (3) (3)
Net actuarial loss (gain) 1 1 1
Net amount recognized in OCI $ (2) $ (1) $ 2