Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	$ (13.4)	$ (4.2)	$ (81.0)	$ 69.9
Change in net unrealized gain during period, tax expense (benefit)	(3.3)	(1.1)	(20.5)	18.7
Change in net unrealized gain during period, net of tax	(10.1)	(3.1)	(60.5)	51.2
Reclassification adjustment for net gains included in income, pre-tax	(0.3)	(0.1)	(0.2)	(0.1)
Reclassification adjustment for net gains included in income, tax expense (benefit)	(0.1)	0.0	(0.1)	0.0
Reclassification adjustment for net gains included in income, net of tax	(0.2)	(0.1)	(0.1)	(0.1)
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, pre-tax	(2.5)	0.0	22.3	0.0
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	(0.6)	0.0	5.7	0.0
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, net of tax	(1.9)	0.0	16.6	0.0
Unrealized loss on derivatives, pre-tax	3.2	0.0	2.3	0.2
Unrealized loss on derivatives, tax expense (benefit)	0.8	0.0	0.6	0.0
Unrealized loss on derivatives, net of tax	2.4	0.0	1.7	0.2	$ (0.2)
Defined benefits post-retirement benefit plan:
Change in net actuarial (gain) loss, pre-tax	0.0	(0.1)	0.0	(0.5)
Change in net actuarial (gain) loss, tax expense (benefit)	0.0		0.0	(0.1)
Change in net actuarial (gain) loss, net of tax	0.0	(0.1)	0.0	(0.4)
Other comprehensive income (loss), before tax	(13.0)	(4.4)	(56.6)	69.5
Other Comprehensive Income (Loss), tax expense (benefit)	(3.2)	(1.1)	(14.3)	18.6
Other Comprehensive Income (Loss), Net of Tax	$ (9.8)	$ (3.3)	$ (42.3)	$ 50.9