Derivatives and Hedging Activities - Pre-tax Gains and Losses on Derivative Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Designated as Hedging Instrument
Derivative [Line Items]
Amount of loss recognized in other comprehensive income (effective portion)	$ (1.1)	$ (0.2)	$ 0.2
Reclassification adjustment for derivative net (gains) losses included in income	1.1	0.0	0.0
Not Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Amount of gain (loss) recognized in other non-interest income	0.0	0.1	0.0
Amount of net fee income recognized in other non-interest income	0.8	0.9	0.1
Amount of net gains (losses) recognized in mortgage banking revenues	$ (1.7)	$ 1.4	$ 0.0