Short-Term Borrowings and Long-Term Debt - Short-term Borrowings (Details) $ in Billions	Mar. 31, 2021 USD ($)
Short-term FHLB advances
Debt Disclosure [Line Items]
FHLB advances	$ 6.9
Unused borrowing capacity	5.8
Federal Reserve Bank Advances
Debt Disclosure [Line Items]
Line of credit, remaining borrowing capacity	0.9
Federal funds lines
Debt Disclosure [Line Items]
Line of credit, remaining borrowing capacity	1.9
Securities sold under agreement to repurchase
Debt Disclosure [Line Items]
Line of credit, remaining borrowing capacity	$ 4.0