Securities (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) Investment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment security gains (losses), net	$ 0.2	$ 0.3
Changes in other-than-temporary-impairment (OTTI) losses	(0.2)	(0.1)
Gross proceeds from sale of securities	$ 2.0	$ 541.3
Number of securities in an unrealized loss position | Investment	1,334
Total fair value	$ 30,546.0		$ 27,562.0
Total unrealized losses	364.0		$ 249.4
Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	147.9
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 29.6
Percent of corporate debt portfolio	38.00%
Other | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 49.1
Auction Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 0.1