Derivative Financial Instruments Offsetting Derivative Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Jul. 31, 2013	Apr. 30, 2013
Offsetting Assets and Liabilities [Line Items]
Gross Amount of Derivative Assets	$ 23	$ 10
Gross Amount of Derivative Liabilities Offset Against Derivative Assets in Balance Sheet	(6)	(5)
Net Amount of Derivative Assets Presented in Balance Sheet	17	5
Gross Amount of Derivative Liabilities Not Offset Against Derivative Assets in Balance Sheet	0	(2)
Net Amount of Derivative Assets	17	3
Gross Amount of Derivative Liabilities	(6)	(9)
Gross Amount of Derivative Assets Offset Against Derivative Liabilities in Balance Sheet	6	5
Net Amount of Derivative Liabilities Presented in Balance Sheet	0	4
Gross Amount of Derivative Assets Not Offset Against Derivative Liabilities in Balance Sheet	0	2
Net Amount of Derivative Liabilities	$ 0	$ 2