Debt Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Derivative, Amount of Hedged Item	$ 208	$ 315
Secured Debt	36,837	43,129
Long-term Debt	44,226	54,128
Subordinated Debt	1,400
Variable Interest Entity Debt	10,200	13,300
Long-term Debt, Excluding Current Maturities	33,037	39,260
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Amount	12,820	17,155
Long-term Debt, Percentage Bearing Variable Interest, Amount	1	1
Trust preferred securities	2,570	2,568
Derivative, Amount of Hedged Item	240	326
Unsecured Debt	$ 15,631	$ 20,050
Debt, Weighted Average Interest Rate	5.68%	5.36%
Long-term Debt	$ 15,631	$ 20,050
Long-term Debt, Excluding Current Maturities	11,909	15,450
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Amount	18,845	17,935
Long-term Debt, Percentage Bearing Variable Interest, Amount	9,782	16,154
Derivative, Amount of Hedged Item	$ (32)	$ (11)
Debt, Weighted Average Interest Rate	1.96%	1.53%
Secured Debt	$ 28,595	$ 34,078
Long-term Debt	28,595	34,078
Long-term Debt Without a Stated Interest Rate	8
Long-term Debt, Excluding Current Maturities	21,128	23,810
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term Debt, Excluding Current Maturities	8,100	14,800
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Secured Debt	$ 10,300	$ 6,100