Pledged Assets (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 41,500,000,000		$ 41,500,000,000		$ 39,600,000,000
Securities eligible to be repledged or sold	117,600,000		117,600,000		151,500,000
Fair value of accepted collateral	354,500,000		354,500,000		426,200,000
Amount of securities re pledged or sold collateral	0		0		0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	1,500,000,000	$ 1,700,000,000	1,500,000,000	$ 1,800,000,000
US Government Agencies Debt Securities and Other Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	32,900,000,000		32,900,000,000		30,900,000,000
Obligations of States and Political Subdivisions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	540,600,000		540,600,000		640,400,000
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	8,100,000,000		8,100,000,000		8,100,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	9,100,000,000		9,100,000,000		9,300,000,000
Available for Sale Debt Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	117,500,000		117,500,000		151,500,000
Repurchase Agreements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	16,500,000		16,500,000		29,000,000.0
Fair value of accepted collateral	300,000,000.0		300,000,000.0		605,000,000.0
Derivative contract
Financial Instruments Owned and Pledged as Collateral [Line Items]
Fair value of accepted collateral	15,600,000		15,600,000		15,300,000
Amount of securities re pledged or sold collateral	$ 0		$ 0		$ 0