Pension and Other Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012	Oct. 31, 2013	Oct. 31, 2012
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 5	$ 5	$ 11	$ 10
Interest cost	8	9	15	17
Expected return on plan assets	(10)	(10)	(20)	(20)
Amortization of:
Net actuarial loss	8	7	16	14
Net cost	11	11	22	21
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:
Net cost	$ 1	$ 1	$ 3	$ 3