Debt (Long-term Debt Portfolio) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 14,553	$ 11,256
Long-term Debt, Excluding Current Maturities	39,260	54,647
Derivative, Amount of Hedged Item	315	331
Long-term debt	54,128	66,234
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	4,274	1,829
Long-term Debt, Excluding Current Maturities	15,450	18,803
Derivative, Amount of Hedged Item	326	334
Long-term debt	20,050	20,966
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	10,279	9,427
Long-term Debt, Excluding Current Maturities	23,810	35,844
Derivative, Amount of Hedged Item	(11)	(3)
Long-term debt	$ 34,078	$ 45,268