Debt (Schedule of Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 10,981	$ 14,553
Long-term Debt, Excluding Current Maturities	33,037	39,260
Derivative, Amount of Hedged Item	208	315
Long-term debt	44,226	54,128
Derivative Instrument Converting Fixed Rated Debt into Variable Rate Debt	3,800	2,100
Derivative Instrument Converting Variable Rate debt into Fixed Rate	3,400	132
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	3,482	4,274
Long-term Debt, Excluding Current Maturities	11,909	15,450
Derivative, Amount of Hedged Item	240	326
Long-term debt	15,631	20,050
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	7,499	10,279
Long-term Debt, Excluding Current Maturities	21,128	23,810
Derivative, Amount of Hedged Item	(32)	(11)
Long-term debt	$ 28,595	$ 34,078