Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2017	Jan. 31, 2016
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 6	$ 6	$ 19	$ 19
Interest cost	9	9	26	26
Expected return on plan assets	(10)	(10)	(31)	(30)
Amortization of:
Prior service cost (credit)	0	0	1	1
Net actuarial loss	6	7	19	21
Settlement loss	1	0	1	0
Net cost	12	12	35	37
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	2	2
Amortization of:
Prior service cost (credit)	(1)	(1)	(2)	(2)
Net actuarial loss	0	0	0	1
Net cost	$ 0	$ 0	$ 1	$ 2