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Pension and Other Postretirement Benefits (Schedule of Components of Other Postretirement Benefit Expense) (Details) - Medical and Life Insurance Benefits [Member] - USD ($)
$ in Millions
12 Months Ended
Apr. 30, 2019
Apr. 30, 2018
Apr. 30, 2017
Postretirement medical and life insurance benefit expense      
Service cost $ 1 $ 1 $ 1
Interest cost 2 1 2
Amortization of prior service cost (credit) (3) (3) (3)
Amortization of net actuarial loss (gain) 1 1 1
Net cost $ 1 $ 0 $ 1