Derivatives, Hedging Activities and Interest Expense - Components of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments Gain Loss [Line Items]
Interest expense on debt									$ 2,559	$ 1,970	$ 1,570
Interest income on derivatives									(53)	(67)	(18)
Interest expense on debt and derivatives									2,506	1,903	1,552
Total interest expense	$ 664	$ 699	$ 702	$ 682	$ 523	$ 428	$ 452	$ 448	2,747	1,851	1,754
Interest expense [Member]
Derivative Instruments Gain Loss [Line Items]
(Gains) losses on non-hedge accounting debt denominated in foreign currencies									(1,078)	1,344	(652)
Losses (gains) on non-hedge accounting foreign currency swaps									1,015	(1,306)	880
Losses (gains) on U.S. dollar non-hedge accounting interest rate swaps									$ 304	$ (90)	$ (26)