Fair Value Measurements - Financial Assets and Liabilities at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	$ 570
Derivatives designated as hedging instruments : Liabilities	670
Available-for-sale Securities, Equity Securities	697	$ 751
Available-for-sale Securities	11,155	5,310
Total Gross Assets	598	463
Credit Support Agreement (CSA)	(290)	(76)
Total Net Asset	308	387
Total Gross Liabilities	715	605
Credit Support Agreement (CSA)	(443)	(238)
Total Net Liabilities	272	367
Quoted prices in active markets for identical assets and liabilities Level 1
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	0
Derivatives designated as hedging instruments : Liabilities	0
Available-for-sale Securities, Equity Securities	697	751
Available-for-sale Securities	0
Significant other observable inputs Level 2
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	570	425
Derivatives designated as hedging instruments : Liabilities	670	567
Available-for-sale Securities, Equity Securities	0
Available-for-sale Securities	11,155
Significant unobservable inputs Level 3
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	0
Derivatives designated as hedging instruments : Liabilities	0
Available-for-sale Securities, Equity Securities	0
Available-for-sale Securities	0
Interest Rate Contract [Member]
Financial assets and liabilities at fair value
Derivative, Notional Amount	1,100	1,100
Derivatives designated as hedging instruments : Assets	19
Derivatives designated as hedging instruments : Liabilities	269
Derivative Assets, Noncurrent	4	7
Derivative Liabilities, Noncurrent - interest swap	6	9
Interest Rate Contract [Member] | Quoted prices in active markets for identical assets and liabilities Level 1
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	0
Derivatives designated as hedging instruments : Liabilities	0
Interest Rate Contract [Member] | Significant other observable inputs Level 2
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	19	7
Derivatives designated as hedging instruments : Liabilities	269	165
Interest Rate Contract [Member] | Significant unobservable inputs Level 3
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	0
Derivatives designated as hedging instruments : Liabilities	0
Foreign exchange contracts
Financial assets and liabilities at fair value
Derivative, Notional Amount	38,800	34,500
Derivatives designated as hedging instruments : Assets	551
Derivatives designated as hedging instruments : Liabilities	401
Derivatives not designated as hedging instruments : Assets	28
Derivatives not designated as hedging instruments : Liabilities	45
Foreign exchange contracts | Quoted prices in active markets for identical assets and liabilities Level 1
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	0
Derivatives designated as hedging instruments : Liabilities	0
Derivatives not designated as hedging instruments : Assets	0
Derivatives not designated as hedging instruments : Liabilities	0
Foreign exchange contracts | Significant other observable inputs Level 2
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	551	418
Derivatives designated as hedging instruments : Liabilities	401	402
Derivatives not designated as hedging instruments : Assets	28	38
Derivatives not designated as hedging instruments : Liabilities	45	38
Foreign exchange contracts | Significant unobservable inputs Level 3
Financial assets and liabilities at fair value
Derivatives designated as hedging instruments : Assets	0
Derivatives designated as hedging instruments : Liabilities	0
Derivatives not designated as hedging instruments : Assets	0
Derivatives not designated as hedging instruments : Liabilities	0
Cross currency interest rate swaps
Financial assets and liabilities at fair value
Derivative, Notional Amount	7,300	2,300
Equity Securities
Financial assets and liabilities at fair value
Available-for-sale Securities, Equity Securities	$ 697	$ 751