Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2016	Oct. 31, 2015	Oct. 31, 2016	Oct. 31, 2015
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 6	$ 6	$ 13	$ 13
Interest cost	9	9	18	17
Expected return on plan assets	(10)	(10)	(21)	(20)
Amortization of:
Net actuarial loss	6	7	13	14
Net cost	11	12	23	24
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	1
Amortization of:
Prior service cost	(1)	(1)	(1)	(1)
Net cost	$ 0	$ 0	$ 1	$ 1