Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	$ (236.4)	$ (13.4)	$ (650.6)	$ (81.0)
Change in net unrealized gain during period, tax expense (benefit)	(53.4)	(3.3)	(161.3)	(20.5)
Change in net unrealized gain during period, net of tax	(183.0)	(10.1)	(489.3)	(60.5)
Reclassification adjustment for net gains included in income, pre-tax	24.2	(0.3)	24.4	(0.2)
Reclassification adjustment for net gains included in income, tax expense (benefit)	5.5	(0.1)	5.5	(0.1)
Reclassification adjustment for net gains included in income, net of tax	18.7	(0.2)	18.9	(0.1)
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, pre-tax	(1.7)	(2.5)	(25.6)	22.3
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	(0.4)	(0.6)	(6.3)	5.7
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, net of tax	(1.3)	(1.9)	(19.3)	16.6
Unrealized loss on derivatives, pre-tax	(15.9)	3.2	(9.6)	2.3
Unrealized loss on derivatives, tax expense (benefit)	(3.6)	0.8	(2.5)	0.6
Unrealized loss on derivatives, net of tax	(12.3)	2.4	(7.1)	1.7
Other comprehensive loss, before tax	(229.8)	(13.0)	(661.2)	(56.6)
Other Comprehensive Income (Loss), tax expense (benefit)	(51.9)	(3.2)	(164.5)	(14.3)
Other Comprehensive Income (Loss), Net of Tax	$ (177.9)	$ (9.8)	(496.7)	(42.3)
Other than Temporary Impairment Losses, Investments, Reclassification Adjustment of Noncredit Portion from Held-to-maturity to Available-for-sale Securities, before Tax			$ 0.2	$ 0.0