Derivatives, Hedging Activities and Interest Expense - Components of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments Gain Loss [Line Items]
Interest expense on debt									$ 2,488	$ 2,559	$ 1,970
Interest expense (income) on derivatives									180	(53)	(67)
Interest expense on debt and derivatives									2,668	2,506	1,903
Total interest expense	$ 769	$ 755	$ 613	$ 697	$ 664	$ 699	$ 702	$ 682	2,834	2,747	1,851
Interest expense [Member]
Derivative Instruments Gain Loss [Line Items]
(Gains) losses on debt denominated in foreign currencies									(703)	(1,078)	1,344
Losses (gains) on foreign currency swaps									650	1,015	(1,306)
Losses (gains) on U.S. dollar interest rate swaps									$ 219	$ 304	$ (90)