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Derivative Financial Instruments and Hedging Activities - Derivatives Income (Losses) Recognized in Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivatives, Fair Value [Line Items]
Total		$ 108		$ 217
Interest expense	$ 838	672	$ 2,373	1,903
Other operating expenses	130	122	433	388
Cash Flow Hedging [Member]
Derivatives, Fair Value [Line Items]
Gains (Losses) Recognized In Accumulated Other Comprehensive Loss	(10)	24	(45)	46
Gains (Losses) Reclassified From Accumulated Other Comprehensive Loss Into Income	11	(27)	64	(60)
Cash Flow Hedging [Member] \| Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Gains (Losses) Recognized In Accumulated Other Comprehensive Loss	0	0	5	1
Gains (Losses) Reclassified From Accumulated Other Comprehensive Loss Into Income	(2)	(1)	(5)	0
Cash Flow Hedging [Member] \| Foreign Currency Contracts [Member]
Derivatives, Fair Value [Line Items]
Gains (Losses) Recognized In Accumulated Other Comprehensive Loss	(10)	24	(50)	45
Gains (Losses) Reclassified From Accumulated Other Comprehensive Loss Into Income	13	(26)	69	(60)
Designated as Hedging Instrument [Member] \| Fair Value Hedging [Member] \| Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts and Foreign currency contracts		9		38
Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts and Foreign currency contracts		2		1
Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Foreign Currency Contracts [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts and Foreign currency contracts		44		99
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts		16		7
Foreign currency derivatives		$ 37		$ 72
Other Operating Income (Expense) [Member]
Derivatives, Fair Value [Line Items]
Total	(28)		(183)
Other Operating Income (Expense) [Member] \| Designated as Hedging Instrument [Member] \| Fair Value Hedging [Member] \| Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Hedged items	0		0
Interest rate contracts and Foreign currency contracts	0		0
Other Operating Income (Expense) [Member] \| Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts and Foreign currency contracts	0		0
Other Operating Income (Expense) [Member] \| Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Foreign Currency Contracts [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts and Foreign currency contracts	(23)		(91)
Other Operating Income (Expense) [Member] \| Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts	0		0
Foreign currency derivatives	(5)		(92)
Interest Expense [Member]
Derivatives, Fair Value [Line Items]
Total	(37)		(76)
Interest Expense [Member] \| Designated as Hedging Instrument [Member] \| Fair Value Hedging [Member] \| Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Hedged items	68		345
Interest rate contracts and Foreign currency contracts	(73)		(359)
Interest Expense [Member] \| Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Interest Rate Contracts [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts and Foreign currency contracts	4		11
Interest Expense [Member] \| Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Foreign Currency Contracts [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts and Foreign currency contracts	(14)		(36)
Interest Expense [Member] \| Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Interest rate contracts	(7)		(2)
Foreign currency derivatives	$ (15)		$ (35)