Fair Value (Changes in Estimated Fair Value for Financial Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Securities — AFS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 67.1	$ 65.9
Included in earnings	0.0	0.0
Included in comprehensive income	(1.4)	1.3
Ending Balance	65.7	67.2
Derivative Assets — Non-Qualifying
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0.3	0.4
Included in earnings	1.2	(0.1)
Included in comprehensive income	0.0	0.0
Ending Balance	1.5	0.3
FDIC True-up Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(68.8)	(66.9)
Included in earnings	(0.5)	(0.5)
Included in comprehensive income	0.0	0.0
Ending Balance	(69.3)	(67.4)
Derivative Liabilities — Non-Qualifying
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(0.8)	0.0
Included in earnings	(1.8)	(0.1)
Included in comprehensive income	0.0	0.0
Ending Balance	$ (2.6)	$ (0.1)