Derivative Financial Instruments and Hedging Activities Offsetting Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Jul. 31, 2017	Apr. 30, 2017
Offsetting Assets and Liabilities [Line Items]
Gross Amount of Derivative Assets	$ 29	$ 35
Gross Amount of Derivative Liabilities Offset Against Derivative Assets in Balance Sheet	(20)	(10)
Net Amount of Derivative Assets Presented in Balance Sheet	9	25
Gross Amount of Derivative Liabilities Not Offset Against Derivative Assets in Balance Sheet	(2)	(1)
Net Amount of Derivative Assets	7	24
Gross Amount of Derivative Liabilities	(48)	(20)
Gross Amount of Derivative Assets Offset Against Derivative Liabilities in Balance Sheet	20	10
Net Amount of Derivative Liabilities Presented in Balance Sheet	28	10
Gross Amount of Derivative Assets Not Offset Against Derivative Liabilities in Balance Sheet	2	1
Net Amount of Derivative Liabilities	$ 26	$ 9