Derivative Financial Instruments (Location and Amount of Derivative Gains and Losses Recorded in Income) (Details) - Asset And Liability Management - Fair Value Hedges - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	$ 2.3	$ 8.2	$ (10.2)	$ 19.4
Available for Sale Investment Securities | Interest Rate Swap Contracts | Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	1.1	15.5	(24.5)	(44.3)
Senior Notes and Long-Term Subordinated Debt | Interest Rate Swap Contracts | Interest Expense
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	$ 1.2	$ (7.3)	$ 14.3	$ 63.7