Borrowings - Short-term Borrowings (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Short-term borrowings	$ 5,700	$ 18,218
Federal Funds Purchased
Short-term Debt [Line Items]
Short-term borrowings	72	259
Securities Sold under Agreements to Repurchase
Short-term Debt [Line Items]
Short-term borrowings	1,123	1,969
FHLB Advances
Short-term Debt [Line Items]
Short-term borrowings	2,410	13,480
Dealer Collateral
Short-term Debt [Line Items]
Short-term borrowings	477	682
Master Notes
Short-term Debt [Line Items]
Short-term borrowings	783	493
Other Short-term Borrowings
Short-term Debt [Line Items]
Short-term borrowings	$ 835	$ 1,335