Derivative Financial Instruments and Hedging Activities Offsetting Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Jan. 31, 2018	Apr. 30, 2017
Offsetting Assets and Liabilities [Line Items]
Gross Amount of Derivative Assets	$ 8	$ 35
Gross Amount of Derivative Liabilities Offset Against Derivative Assets in Balance Sheet	(6)	(10)
Net Amount of Derivative Assets Presented in Balance Sheet	2	25
Gross Amount of Derivative Liabilities Not Offset Against Derivative Assets in Balance Sheet	(2)	(1)
Net Amount of Derivative Assets	0	24
Gross Amount of Derivative Liabilities	(75)	(20)
Gross Amount of Derivative Assets Offset Against Derivative Liabilities in Balance Sheet	6	10
Net Amount of Derivative Liabilities Presented in Balance Sheet	69	10
Gross Amount of Derivative Assets Not Offset Against Derivative Liabilities in Balance Sheet	2	1
Net Amount of Derivative Liabilities	$ 67	$ 9