Pension and Other Postretirement Benefits (Schedule of Components of Pension Expense) (Details) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Pension Expense
Service cost	$ 24	$ 24	$ 24
Interest cost	31	34	29
Expected return on assets	(46)	(47)	(41)
Amortization of prior service cost (credit)	1	1	1
Amortization of net actuarial loss (gain)	19	19	21
Settlement charge	1	15	0
Net cost	$ 30	$ 46	$ 34