Securities (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Investment	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Investment	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Gains (Losses), net (Note)	$ 0.0	$ (0.3)	$ 0.3	$ 3.3
Gross proceeds from sale of securities	3.8	337.3	110.3	801.2
Gross realized securities gains		1.0	0.1	1.7
Gross realized securities losses		0.7	0.4	1.1
Net impairment losses recognized in earnings	$ 0.0	$ 0.0	$ 0.0	$ 3.9
Number of securities in an unrealized loss position | Investment	581		581
Total fair value	$ 11,701.2		$ 11,701.2		$ 10,410.8
Total unrealized losses	74.8		74.8		$ 87.6
Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	12.7		12.7
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 20.1		$ 20.1
Percent of corporate debt portfolio	35.00%		35.00%
Other | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 34.1		$ 34.1
Auction Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 1.0		$ 1.0
Other Asset-Backed | Floating Rate Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of "other asset-backed" securities rated triple-A	99.60%		99.60%
Other Asset-Backed | Floating Rate Securities | Upper Limit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average life			5 years