Other Real Estate Owned (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Repossessed Assets [Abstract]
Real Estate Acquired Through Foreclosure, Acquired Through Acquisition	$ 0.0	$ 0.0	$ 0.0	$ 2.4
Real Estate Acquired Through Foreclosure [Roll Forward]
Beginning balance	6.5	27.5	8.5	14.4
Additions	2.0	0.7	3.2	14.0
Valuation adjustments	(0.1)	(0.2)	(0.1)	(0.7)
Dispositions	(2.7)	(10.2)	(5.9)	(12.3)
Ending balance	5.7	$ 17.8	5.7	$ 17.8
Carrying values of foreclosed residential real estate properties	2.3		2.3		$ 2.3
Consumer mortgage loans collateralized by residential real estate property in the process of foreclosure	0.2		0.2		1.4
Other Real Estate, Foreclosed Assets, and Repossessed Assets	$ 2.3		$ 2.3		$ 2.3