Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	$ (66.8)	$ 39.2
Change in net unrealized gain during period, tax expense (benefit)	(17.0)	10.3
Change in net unrealized gain during period, net of tax	(49.8)	28.9
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	(0.1)	0.0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax	0.0	0.0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	(0.1)	0.0
Defined benefits post-retirement benefit plan:
Change in net actuarial (gain) loss, pre-tax	0.0	(0.2)
Change in net actuarial (gain) loss, tax expense (benefit)	0.0	(0.1)
Change in net actuarial (gain) loss, net of tax	0.0	(0.1)
Other comprehensive (loss) income, before tax	(66.9)	39.0
Other Comprehensive Income (Loss), Tax	(17.0)	10.2
Other Comprehensive Income (Loss), Net of Tax	$ (49.9)	$ 28.8