Derivative Financial Instruments and Hedging Activities Offsetting Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Jul. 31, 2018	Apr. 30, 2018
Offsetting Assets and Liabilities [Line Items]
Gross Amount of Derivative Assets	$ 18	$ 10
Gross Amount of Derivative Liabilities Offset Against Derivative Assets in Balance Sheet	(13)	(9)
Net Amount of Derivative Assets Presented in Balance Sheet	5	1
Gross Amount of Derivative Liabilities Not Offset Against Derivative Assets in Balance Sheet	(1)	(1)
Net Amount of Derivative Assets	4	0
Gross Amount of Derivative Liabilities	(20)	(48)
Gross Amount of Derivative Assets Offset Against Derivative Liabilities in Balance Sheet	13	9
Net Amount of Derivative Liabilities Presented in Balance Sheet	7	39
Gross Amount of Derivative Assets Not Offset Against Derivative Liabilities in Balance Sheet	1	1
Net Amount of Derivative Liabilities	$ 6	$ 38