Other Real Estate Owned (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Real Estate Acquired Through Foreclosure, Acquired Through Acquisition	$ 0.6	$ 0.0	$ 0.6	$ 1.2
Real Estate Acquired Through Foreclosure [Roll Forward]
Beginning balance	14.9	11.3	10.1	10.0
Additions	3.2	1.4	10.6	3.2
Valuation adjustments	(0.1)	(0.2)	(0.1)	(0.3)
Dispositions	(1.3)	(2.1)	(3.9)	(3.7)
Ending balance	17.3	$ 10.4	17.3	$ 10.4
Consumer mortgage loans collateralized by residential real estate property in the process of foreclosure	2.0		2.0		$ 0.0
Residential real estate
Real Estate Acquired Through Foreclosure [Roll Forward]
Beginning balance			2.7
Ending balance	$ 2.6		$ 2.6