Pension and Other Postretirement Benefits (Schedule of Components of Pension Expense) (Details) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Pension Expense
Service cost	$ 24	$ 26	$ 26
Interest cost	29	35	35
Expected return on assets	(41)	(41)	(40)
Amortization of prior service cost (credit)	1	1	1
Amortization of net actuarial loss (gain)	21	25	27
Settlement loss	0	1	0
Net cost	$ 34	$ 47	$ 49