Pension and Other Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Jan. 31, 2015	Jan. 31, 2014	Jul. 31, 2012	Jan. 31, 2015	Jan. 31, 2014
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 5	$ 5		$ 16	$ 16
Interest cost	8	8		25	23
Expected return on plan assets	(10)	(10)		(31)	(30)
Amortization of:
Prior service cost	0	0		1	1
Net actuarial loss	6	8		17	23
Net cost	9	11		28	33
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	3
Amortization of:
Prior service cost	(1)	0		(1)	0
Net actuarial loss	0		0	1
Net cost	$ 0	$ 1		$ 3	$ 4