Securities (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) Investment	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Investment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Held to maturity securities, transferred from available for sale	$ 287.9	$ 1,000.0	$ 287.9	$ 1,000.0
Investment Security Losses, net	(0.1)	(0.4)	(0.3)	(1.1)
Gross proceeds from sale of securities	0.0	398.8	178.6	1,900.0
OTTI losses	$ 0.1	$ 0.0	0.3	0.1
Realized gain			1.5	0.2
Realized loss			$ 1.5	$ 1.3
Number of securities in an unrealized loss position | Investment	1,432		1,432
Total fair value	$ 34,200.0		$ 34,200.0
Total unrealized losses	430.5		430.5
Debt securities, unrealized losses	430.5		430.5		$ 249.4
Community Reinvestment Act CRA
Debt and Equity Securities, FV-NI [Line Items]
Investment Security Losses, net			(0.3)
CRA Eligible, Mortgage Backed Securities
Debt and Equity Securities, FV-NI [Line Items]
OTTI losses	0.1
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	199.6		199.6
Debt securities, unrealized losses	199.6		199.6		108.6
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	27.7		27.7
Debt securities, unrealized losses	$ 27.7		$ 27.7		23.3
Percent of corporate debt portfolio	38.00%		38.00%
Other
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	$ 57.1		$ 57.1
Debt securities, unrealized losses	$ 57.1		$ 57.1		$ 45.2