Derivatives and Hedging Activities - Pre-tax Gains and Losses on Derivative Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Amount of loss recognized in other comprehensive income (effective portion)	$ (0.2)
Designated as Hedging Instrument
Derivative [Line Items]
Amount of loss recognized in other comprehensive income (effective portion)	0.1	$ 0.0	$ 0.0
Reclassification adjustment for derivative net gain included in income	(0.1)	0.0	0.0
Designated as Hedging Instrument | Interest Rate Swap | Cash Flow Hedging
Derivative [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	0.0
Not Designated as Hedging Instrument
Derivative [Line Items]
Amount of gain recognized in other non-interest income	0.0	0.0
Amount of net fee income recognized in other non-interest income	7.7	2.5
Gain (Loss) on Fair Value Hedges Recognized in Earnings	$ 1.2	$ 0.3
Not Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Amount of gain recognized in other non-interest income			0.3
Amount of net fee income recognized in other non-interest income			1.3
Amount of net gains (losses) recognized in mortgage banking revenues			$ (0.5)