Debt (Long-term Debt) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt, due within one year	$ 7,688	$ 9,219
Long-term debt, due after one year	23,378	24,808
Total long-term debt	31,066	34,027
Trust preferred securities	2,600	2,600
Secured Debt	28,538	25,773
Unsecured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	477	2,214
Long-term debt, due after one year	9,167	8,990
Total long-term debt	9,644	11,204
Secured debt
Debt Instrument [Line Items]
Long-term debt, due within one year	7,211	7,005
Long-term debt, due after one year	14,211	15,818
Total long-term debt	21,422	22,823
Federal Home Loan Bank advances
Debt Instrument [Line Items]
Secured Debt	$ 12,100	$ 13,300