Derivatives, Hedging Activities and Interest Expense - Components of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments Gain Loss [Line Items]
Interest expense on debt	$ 620	$ 652	$ 1,914	$ 1,890
Interest expense (income) on derivatives	51	(4)	100	(51)
Interest expense on debt and derivatives	671	648	2,014	1,839
Total interest expense	755	699	2,065	2,083
Interest expense [Member]
Derivative Instruments Gain Loss [Line Items]
Losses (gains) on debt denominated in foreign currencies	452	(178)	(8)	(985)
(Gains) losses on foreign currency swaps	(385)	140	(19)	995
Losses on U.S. dollar interest rate swaps	$ 17	$ 89	$ 78	$ 234