Borrowings - Short-term Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Short-term borrowings	$ 18,218	$ 5,178
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.67%	2.45%
Federal Funds Purchased
Short-term Debt [Line Items]
Short-term borrowings	$ 259	$ 0
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.48%	0.00%
Securities Sold under Agreements to Repurchase
Short-term Debt [Line Items]
Short-term borrowings	$ 1,969	$ 251
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.41%	1.59%
FHLB Advances
Short-term Debt [Line Items]
Short-term borrowings	$ 13,480	$ 4,525
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.87%	2.55%
Dealer Collateral
Short-term Debt [Line Items]
Short-term borrowings	$ 682	$ 0
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.55%	0.00%
Master Notes
Short-term Debt [Line Items]
Short-term borrowings	$ 493	$ 0
Short-term Debt, Weighted Average Interest Rate, at Point in Time	0.94%	0.00%
Other Short-term Borrowings
Short-term Debt [Line Items]
Short-term borrowings	$ 1,335	$ 402
Short-term Debt, Weighted Average Interest Rate, at Point in Time	0.44%	1.67%