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, 2018	Apr. 30, 2017	Apr. 30, 2016
Postretirement medical and life insurance benefit expense
Service cost	$ 1	$ 1	$ 1
Interest cost	1	2	2
Amortization of prior service cost (credit)	(3)	(3)	(2)
Amortization of net actuarial loss (gain)	1	1	1
Net cost	$ 0	$ 1	$ 2