Derivatives and Hedging Activities - Schedule of Derivative Instruments, Effect on Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Jun. 17, 2021	Dec. 31, 2020
Derivative [Line Items]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	$ (0.8)	$ 0.2	$ (0.9)	$ 0.2
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	0.1		0.1
Derivative Instruments and Hedges, Assets	500.8		500.8			$ 0.0
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	0.8		0.8			0.0
Derivative, Fair Value, Net	11.8		11.8
Financial Instruments	0.0		0.0			0.0
Collateral Pledged
Derivative [Line Items]
Financial Instruments	0.2		0.2
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Fixed Interest Rate					1.19%
Derivative, Notional Amount					$ 500.0
Not Designated as Hedging Instrument
Derivative [Line Items]
Amount of net fee income recognized in other non-interest income	0.1	1.7	1.2	2.7
Amount of net gains recognized in mortgage banking revenues	0.2	0.5	0.5	5.0
Interest Rate Swap [Member]
Derivative [Line Items]
Financial Instruments	0.0		0.0			$ 0.0
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, Tax	$ 0.0	$ 0.0	$ 0.0	$ 0.0