Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year	$ 155		$ 155
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US	country:US	country:US
Service cost	$ 6	$ 6	$ 18	$ 19
Interest cost	7	9	22	26
Expected return on plan assets	(10)	(10)	(31)	(31)
Amortization of:
Prior service cost (credit)	0	0	0	1
Net actuarial loss	6	6	16	19
Settlement loss	0	1	0	1
Net cost	$ 9	$ 12	$ 25	$ 35
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	2
Amortization of:
Prior service cost (credit)	(1)	(1)	(2)	(2)
Net cost	$ 0	$ 0	$ 0	$ 1