Derivatives, Hedging Activities and Interest Expense - Components of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Derivative Instruments Gain Loss [Line Items]
Interest expense on debt									$ 1,954	$ 2,488	$ 2,559
Interest expense (income) on derivatives									420	180	(53)
Interest expense on debt and derivatives									2,374	2,668	2,506
Losses (gains) on debt denominated in foreign currencies									1,402	(703)	(1,078)
(Gains) losses on foreign currency swaps									(1,351)	650	1,015
Total interest expense	$ 768	$ 491	$ 495	$ 548	$ 769	$ 755	$ 613	$ 697	2,302	2,834	2,747
Interest Expense
Derivative Instruments Gain Loss [Line Items]
(Gains) losses on U.S. dollar interest rate swaps									$ (123)	$ 219	$ 304