Short-Term Borrowings and Long-Term Debt - Short-term Borrowings (Details) $ in Billions	Mar. 31, 2020 USD ($)
Short-term FHLB advances
Debt Disclosure [Line Items]
FHLB advances	$ 4.8
Unused borrowing capacity	1.4
Federal Reserve Bank Advances
Debt Disclosure [Line Items]
Line of credit, remaining borrowing capacity	1.0
Federal funds lines
Debt Disclosure [Line Items]
Line of credit, remaining borrowing capacity	1.6
Securities sold under agreement to repurchase
Debt Disclosure [Line Items]
Line of credit, remaining borrowing capacity	$ 3.3