Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Equity [Abstract]
Other Comprehensive Income (Loss), Net of Tax	$ (3.3)	$ 2.9	$ 50.9	$ 38.0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax			0.2	0.0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax			0.0	0.0
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	(4.2)	4.2	69.9	58.0
Change in net unrealized gain during period, tax expense (benefit)	(1.1)	1.0	18.7	15.0
Change in net unrealized gain during period, net of tax	(3.1)	3.2	51.2	43.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.1)	(0.1)	(0.1)	(0.1)
Defined benefits post-retirement benefit plan:
Change in net actuarial (gain) loss, pre-tax	(0.1)	(0.2)	(0.5)	(0.5)
Change in net actuarial (gain) loss, tax expense (benefit)	0.0		(0.1)
Change in net actuarial (gain) loss, net of tax	(0.1)	(0.2)	(0.4)	(0.5)
Other comprehensive (loss) income, before tax	(4.4)	3.9	69.5	51.4
Reclassification adjustment for net gains included in income	(0.1)	(0.1)	(0.1)	(0.1)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	$ 0.0	$ 0.0	0.0	0.0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax			$ 0.2	$ 0.0