Debt (Schedule of Debt) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	$ 11,744	$ 14,553
Long-term Debt, Excluding Current Maturities	37,101	39,260
Derivative, Amount of Hedged Item	300	315
Long-term debt	49,145	54,128
Secured Debt	37,591	43,129
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	3,342	4,274
Long-term Debt, Excluding Current Maturities	13,909	15,450
Derivative, Amount of Hedged Item	309	326
Long-term debt	17,560	20,050
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Current Maturities	8,402	10,279
Long-term Debt, Excluding Current Maturities	23,192	23,810
Derivative, Amount of Hedged Item	(9)	(11)
Long-term debt	31,585	34,078
Trust Preferred Securities Subject to Mandatory Redemption [Member]
Debt Instrument [Line Items]
Long-term Debt, Excluding Current Maturities	2,600	2,600
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Secured Debt	$ 6,500	$ 6,100