Derivatives, Hedging Activities and Interest Expense - Components of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments Gain Loss [Line Items]
Interest expense on debt	$ 639	$ 637	$ 1,294	$ 1,238
Interest expense (income) on derivatives	37	(22)	49	(47)
Interest expense on debt and derivatives	676	615	1,343	1,191
Total interest expense	613	702	1,310	1,384
Interest expense [Member]
Derivative Instruments Gain Loss [Line Items]
Gains on debt denominated in foreign currencies	(475)	(161)	(460)	(807)
Losses on foreign currency swaps	474	172	366	855
(Gains) losses on U.S. dollar interest rate swaps	$ (62)	$ 76	$ 61	$ 145