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
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	$ (5.2)	$ 0.7
Available for Sale Debt Securities | Interest Rate Swap Contracts | Interest Income
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Derivative Gain Recognized in Income	0.5	(2.4)
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	$ (5.7)	$ 3.1