Other Comprehensive Income/Loss - Schedule of 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
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	$ (0.8)	$ 34.9	$ (67.6)	$ 74.1
Change in net unrealized gain during period, tax expense (benefit)	(0.2)	9.5	(17.2)	19.8
Change in net unrealized gain during period, net of tax	(0.6)	25.4	(50.4)	54.3
Reclassification adjustment for net gains included in income, pre-tax	0.1	0.0	0.1	0.0
Reclassification adjustment for net gains included in income, tax expense (benefit)	0.0	0.0	0.0	0.0
Reclassification adjustment for net gains included in income, net of tax	0.1	0.0	0.1	0.0
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, pre-tax	24.8	0.0	24.8	0.0
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	6.3	0.0	6.3	0.0
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, net of tax	18.5	0.0	18.5	0.0
Unrealized gain (loss) on derivatives, pre-tax	(0.8)	0.2	(0.9)	0.2
Unrealized gain (loss) on derivatives, tax expense (benefit)	(0.2)	0.0	(0.2)	0.0
Unrealized gain (loss) on derivatives, net of tax	(0.6)	0.2	(0.7)	0.2
Defined benefits post-retirement benefit plan:
Change in net actuarial (gain) loss, pre-tax	0.0	(0.2)	0.0	(0.4)
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	(0.2)	0.0	(0.3)
Other comprehensive income (loss), before tax	23.3	34.9	(43.6)	73.9
Other Comprehensive Income (Loss), tax expense (benefit)	5.9	9.5	(11.1)	19.7
Other Comprehensive Income (Loss), Net of Tax	$ 17.4	$ 25.4	$ (32.5)	$ 54.2