Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Jul. 31, 2017	Jul. 31, 2016
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US
Service cost	$ 6	$ 6
Interest cost	8	9
Expected return on plan assets	(10)	(10)
Amortization of:
Net actuarial loss	5	6
Net cost	$ 9	$ 11
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US
Interest cost	$ 1	$ 1
Amortization of:
Prior service cost (credit)	(1)	(1)
Net cost	$ 0	$ 0