Pension and Other Postretirement Benefits (Schedule of Components of Other Postretirement Benefit Expense) (Details) (Medical and Life Insurance Benefits [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Apr. 30, 2015	Apr. 30, 2014	Apr. 30, 2013
Medical and Life Insurance Benefits [Member]
Postretirement medical and life insurance benefit expense
Service cost	$ 1	$ 2	$ 2
Interest cost	3	3	3
Amortization of prior service cost (credit)	(2)	0	1
Amortization of net actuarial loss (gain)	1	0	0
Net expense	$ 3	$ 5	$ 6