Pledged Assets (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 40,100,000,000		$ 40,100,000,000		$ 40,100,000,000
Securities eligible to be repledged or sold	34,800,000		34,800,000		833,400,000
Fair value of accepted collateral	728,500,000		728,500,000		78,300,000
Amount of securities re pledged or sold collateral	0		0		0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	1,700,000,000	$ 4,000,000,000	1,700,000,000	$ 2,700,000,000
US Government Agencies Debt Securities And Other Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	31,000,000,000		31,000,000,000		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	721,700,000		721,700,000		684,300,000
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	8,400,000,000		8,400,000,000		8,600,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	9,200,000,000		9,200,000,000		8,900,000,000
Available for Sale Debt Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	31,100,000		31,100,000		833,400,000
Repurchase Agreements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	27,600,000		27,600,000		39,900,000
Fair value of accepted collateral	730,000,000		730,000,000		1,200,000,000
Derivative contract
Financial Instruments Owned and Pledged as Collateral [Line Items]
Fair value of accepted collateral	12,400,000		12,400,000		4,600,000
Amount of securities re pledged or sold collateral	$ 0		$ 0		$ 0