Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Stated interest rate	8.125%
Total secured debt	$ 39,596	$ 36,837
Long-term debt	44,193	44,226
Subordinated debt	1,000	1,400
Variable interest entity debt	10,500	10,200
Long-term debt, due after one year	35,217	33,189
Unsecured debt
Debt Instrument [Line Items]
Long-term debt, fixed interest rate, amount	9,406	12,820
Long-term debt, variable interest rate, amount	1	1
Trust preferred securities	2,572	2,570
Fair value adjustment	128	240
Total unsecured debt	$ 12,107	$ 15,631
Weighted-average stated interest rate	6.29%	5.68%
Long-term debt	$ 12,107	$ 15,631
Long-term debt, due after one year	10,444	12,091
Secured debt
Debt Instrument [Line Items]
Long-term debt, fixed interest rate, amount	23,514	18,845
Long-term debt, variable interest rate, amount	8,633	9,782
Fair value adjustment	$ (61)	$ (32)
Weighted-average stated interest rate	2.54%	1.96%
Total secured debt	$ 32,086	$ 28,595
Long-term debt	32,086	28,595
Long-term debt without a stated interest rate	5	8
Long-term debt, due after one year	24,773	21,098
Revolving credit facilities
Debt Instrument [Line Items]
Long-term debt, due after one year	6,700	8,100
Federal Home Loan Bank advances
Debt Instrument [Line Items]
Total secured debt	$ 14,900	$ 10,300
Minimum | Unsecured debt
Debt Instrument [Line Items]
Stated interest rate	2.42%	1.48%
Minimum | Secured debt
Debt Instrument [Line Items]
Stated interest rate	1.26%	0.63%
Maximum | Unsecured debt
Debt Instrument [Line Items]
Stated interest rate	8.40%	8.00%
Maximum | Secured debt
Debt Instrument [Line Items]
Stated interest rate	4.50%	4.50%