Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Equity [Abstract]
Other Comprehensive Income (Loss), Net of Tax	$ 25.4	$ 14.9	$ 54.2	$ 35.1
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	0.2	0.0	0.2	0.0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	0.0	0.0	0.0	0.0
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	34.9	20.2	74.1	53.8
Change in net unrealized gain during period, tax expense (benefit)	9.5	5.1	19.8	14.0
Change in net unrealized gain during period, net of tax	25.4	15.1	54.3	39.8
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax			0.0	(4.4)
Defined benefits post-retirement benefit plan:
Change in net actuarial (gain) loss, pre-tax	(0.2)	(0.2)	(0.4)	(0.3)
Change in net actuarial (gain) loss, tax expense (benefit)	0.0		(0.1)
Change in net actuarial (gain) loss, net of tax	(0.2)	(0.2)	(0.3)	(0.3)
Other comprehensive income, before tax	34.9	20.0	73.9	47.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax			0.0	(6.0)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax			0.0	(1.6)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	$ 0.2	$ 0.0	$ 0.2	$ 0.0