Derivatives, Hedging Activities and Interest Expense - Components of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments Gain Loss [Line Items]
Interest expense on debt	$ 652	$ 490	$ 1,890	$ 1,441
Interest income on derivatives	(4)	(13)	(51)	(47)
Interest expense on debt and derivatives	648	477	1,839	1,394
Total interest expense	699	428	2,083	1,328
Interest expense [Member]
Derivative Instruments Gain Loss [Line Items]
(Gains) losses on non-hedge accounting debt denominated in foreign currencies	(178)	123	(985)	1,146
Losses (gains) on non-hedge accounting foreign currency swaps	140	(108)	995	(1,141)
Losses (gains) on U.S. dollar non-hedge accounting interest rate swaps	$ 89	$ (64)	$ 234	$ (71)