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, 2017	Apr. 30, 2016	Apr. 30, 2015
Postretirement medical and life insurance benefit expense
Service cost	$ 1	$ 1	$ 1
Interest cost	2	2	3
Amortization of prior service cost (credit)	(3)	(2)	(2)
Amortization of net actuarial loss (gain)	1	1	1
Net expense	$ 1	$ 2	$ 3