Short-Term Borrowings and Long-Term Debt - Short-term Borrowings (Details) $ in Billions	Dec. 31, 2020 USD ($)
Short-term FHLB advances
Short-term Debt [Line Items]
FHLB advances	$ 6.8
Amount of unused FHLB credit facility	5.8
FRB advances
Short-term Debt [Line Items]
Carrying value of collateral pledged	0.9
Line of credit, remaining borrowing capacity	0.9
Uncommitted federal funds lines
Short-term Debt [Line Items]
Line of credit, remaining borrowing capacity	1.9
Master repurchase agreements
Short-term Debt [Line Items]
Line of credit, remaining borrowing capacity	$ 4.0