Pledged Assets (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 42,100,000,000		$ 42,100,000,000		$ 40,300,000,000
Fair value of accepted collateral	1,500,000,000		1,500,000,000		1,800,000,000
Amount of securities re pledged or sold collateral	0		0		0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	1,800,000,000	$ 1,800,000,000	2,100,000,000	$ 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	31,200,000,000		31,200,000,000		29,700,000,000
Obligations of States and Political Subdivisions
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	868,700,000		868,700,000		939,800,000
Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	10,000,000,000		10,000,000,000		9,600,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	7,000,000,000		7,000,000,000		9,700,000,000
Available for Sale Investment Securities
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amount of securities and loans pledged	$ 1,400,000,000		$ 1,400,000,000		$ 1,400,000,000