Borrowings and Contractual Commitments Short-Term Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Federal Funds Purchased	$ 2,141	$ 2,561
Securities Sold under Agreements to Repurchase	1,774	1,503
Other Short-term Borrowings	4,857	717
Short-term Debt	$ 8,772	$ 4,781
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.46%	1.30%
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.40%	1.33%
Securities Sold under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.58%	1.39%
Other Short-Term Borrowings [Member]
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.44%	1.00%
Federal Home Loan Bank Advances [Member]
Short-term Debt [Line Items]
Other Short-term Borrowings	$ 4,000	$ 0
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.53%
Dealer Collateral [Member]
Short-term Debt [Line Items]
Other Short-term Borrowings	$ 503	$ 367
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.40%	1.33%
Master Notes [Member]
Short-term Debt [Line Items]
Other Short-term Borrowings	$ 354	$ 350
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.40%	0.66%