Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 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	country:US	country:US
Service cost	$ 6	$ 6	$ 12	$ 13
Interest cost	7	9	15	18
Expected return on plan assets	(10)	(10)	(21)	(21)
Amortization of:
Net actuarial loss	6	6	11	13
Net cost	$ 9	$ 11	$ 17	$ 23
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	country:US	country:US
Service cost	$ 0	$ 0	$ 0	$ 1
Interest cost	1	1	1	1
Amortization of:
Prior service cost (credit)	(1)	(1)	(1)	(1)
Net cost	$ 0	$ 0	$ 0	$ 1