Securities (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Investment	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Investment	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Gains (Losses), net (Note)	$ 0	$ (400,000)	$ (200,000)	$ (700,000)
Gross proceeds from sale of securities	176,600,000	978,500,000	178,600,000	1,500,000,000
OTTI losses	$ 0	$ 0	200,000	100,000
Realized gain			1,500,000	100,000
Realized loss			$ 1,500,000	$ 800,000
Number of securities in an unrealized loss position | Investment	1,340		1,340
Total fair value	$ 32,000,000,000		$ 32,000,000,000
Total unrealized losses	383,500,000		383,500,000
Debt securities, unrealized losses	383,500,000		383,500,000		$ 249,400,000
Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Gains (Losses), net (Note)			(200,000)
Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	167,800,000		167,800,000
Debt securities, unrealized losses	167,800,000		167,800,000		108,600,000
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	30,100,000		30,100,000
Debt securities, unrealized losses	$ 30,100,000		$ 30,100,000		23,300,000
Percent of corporate debt portfolio	38.00%		38.00%
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities, unrealized losses	$ 53,800,000		$ 53,800,000		45,200,000
Other | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	53,800,000		53,800,000
Auction Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	100,000		100,000
Debt securities, unrealized losses	$ 100,000		$ 100,000		$ 100,000