Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2015	Oct. 31, 2014	Oct. 31, 2015	Oct. 31, 2014
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 6	$ 5	$ 13	$ 11
Interest cost	9	8	17	17
Expected return on plan assets	(10)	(10)	(20)	(21)
Amortization of:
Prior service cost	0	0	0	1
Net actuarial loss	7	6	14	11
Net cost	12	9	24	19
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	1	2
Amortization of:
Prior service cost	(1)	0	(1)	(1)
Net cost	$ 0	$ 1	$ 1	$ 2