Loans - Schedule of Changes in Accretable Yield for Loans Acquired with Credit Impairment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Rollforward [Abstract]
Beginning Balance	$ 7.3	$ 6.8	$ 6.7
Acquisitions	3.2	1.9	1.1
Additions	(3.1)	(2.9)	(2.5)
Accretion income	0.6	0.1	0.0
Reductions due to exit events	(1.1)	(1.5)	(1.1)
Reclassifications from nonaccretable differences	2.0	2.9	2.6
Ending Balance	$ 8.9	$ 7.3	$ 6.8