Other Real Estate Owned (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Loans and Leases Receivable, Troubled Debt Restructuring [Line Items]
Real Estate Acquired Through Foreclosure, Acquired Through Acquisition	$ 0.0	$ 1.1	$ 0.0	$ 1.1
Real Estate Acquired Through Foreclosure [Roll Forward]
Beginning balance	11.0	9.4	10.1	10.0
Additions	6.3	1.4	7.4	1.8
Valuation adjustments	0.0	0.0	0.0	(0.1)
Dispositions	(2.4)	(0.6)	(2.6)	(1.5)
Ending balance	14.9	$ 11.3	14.9	$ 11.3
Consumer mortgage loans collateralized by residential real estate property in the process of foreclosure	2.5		2.5		$ 0.0
Residential real estate
Real Estate Acquired Through Foreclosure [Roll Forward]
Beginning balance			2.7
Ending balance	$ 2.6		$ 2.6