Securities (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Investment	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Investment	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Held to maturity securities, transferred from available for sale	$ 1,000.0
Investment security gains (losses), net	0.4	$ (0.2)	$ 1.1	$ 1.9
Gross proceeds from sale of securities	398.8	217.2	1,900.0	740.8
Changes in other-than-temporary-impairment (OTTI) losses	$ 0.0	$ 0.0	(0.1)	(2.4)
Gross realized securities gains			0.2	$ 0.5
Gross realized securities losses			$ 1.3
Number of securities in an unrealized loss position | Investment	1,130		1,130
Total fair value	$ 23,541.6		$ 23,541.6		$ 22,040.2
Total unrealized losses	159.7		159.7		$ 176.0
Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	61.7		61.7
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 16.6		$ 16.6
Percent of corporate debt portfolio	37.00%		37.00%
Other | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 37.3		$ 37.3
Auction Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 0.3		$ 0.3