Derivatives, Hedging Activities and Interest Expense - Components of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments Gain Loss [Line Items]
Interest expense on debt	$ 601	$ 467
Interest income on derivatives	(25)	(16)
Interest expense on debt and derivatives, net	576	451
Total interest expense	682	448
Interest expense [Member]
Derivative Instruments Gain Loss [Line Items]
(Gain) loss on non-hedge accounting debt denominated in foreign currencies	(646)	650
Loss (gain) on non-hedge accounting foreign currency swaps	683	(650)
Loss (gain) on U.S. dollar non-hedge accounting interest rate swaps	$ 69	$ (3)