Pledged Assets (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 40,800,000,000		$ 40,100,000,000
Fair value of accepted collateral	152,100,000		78,300,000
Amount of securities re pledged or sold collateral			0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	1,800,000,000	$ 2,000,000,000
US Government Agencies Debt Securities And Other Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	31,900,000,000		30,800,000,000
Obligations of States and Political Subdivisions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	708,400,000		684,300,000
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	8,200,000,000		8,600,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	11,300,000,000		8,900,000,000
Available for Sale Debt Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	785,000,000		833,400,000
Repurchase Agreements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Fair value of accepted collateral	1,400,000,000		1,200,000,000
Derivative contract
Financial Instruments Owned and Pledged as Collateral [Line Items]
Fair value of accepted collateral	22,800,000		4,600,000
Amount of securities re pledged or sold collateral	$ 0		$ 0