Pension and Other Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2014	Oct. 31, 2013
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 5	$ 5	$ 11	$ 11
Interest cost	8	8	17	15
Expected return on plan assets	(10)	(10)	(21)	(20)
Amortization of:
Prior service cost	0	0	1	0
Net actuarial loss	6	8	11	16
Net cost	9	11	19	22
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	1	1
Interest cost	1	1	2	2
Amortization of:
Prior service cost	0	0	(1)	0
Net cost	$ 1	$ 1	$ 2	$ 3