Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2019	Jan. 31, 2018
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	$ 18	$ 17
Interest cost	9	7	26	22
Expected return on plan assets	(12)	(10)	(35)	(31)
Amortization of:
Prior service cost (credit)	0	0	1	0
Net actuarial loss	5	5	15	17
Settlement loss	13	0	13	0
Net cost	$ 21	$ 8	$ 38	$ 25
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	$ 1	$ 1
Interest cost	1	1	1	1
Amortization of:
Prior service cost (credit)	(1)	(1)	(2)	(2)
Net cost	$ 0	$ 0	$ 0	$ 0