Loans - Schedule of Changes in Accretable Yield for Loans Acquired with Credit Impairment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Rollforward [Abstract]
Beginning Balance	$ 6.8	$ 6.7	$ 5.8
Acquisitions	1.9	1.1	0.4
Accretion income	(2.9)	(2.5)	(2.9)
Additions	0.1	0.0	0.6
Reductions due to exit events	(1.5)	(1.1)	(0.5)
Reclassifications from nonaccretable differences	2.9	2.6	3.3
Ending Balance	$ 7.3	$ 6.8	$ 6.7