Borrowings and Contractual Commitments Short-Term Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Federal Funds Purchased	$ 2,561	$ 2,116
Other Short-term Borrowings	717	1,015
Securities Sold under Agreements to Repurchase	1,503	1,633
Short-term Debt	$ 4,781	$ 4,764
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.33%	0.55%
Other Liabilities [Member]
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.00%	0.56%
Securities Sold under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.39%	0.55%