Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	$ (57.5)	$ (161.6)	$ 20.7	$ (414.2)
Change in net unrealized gain during period, tax expense (benefit)	(14.4)	(45.2)	5.2	(107.9)
Change in net unrealized gain during period, net of tax	(43.1)	(116.4)	15.5	(306.3)
Reclassification adjustment for net gains included in income, pre-tax	0.1	0.1	23.5	0.2
Reclassification adjustment for net gains included in income, tax expense (benefit)	0.0	0.0	5.8	0.0
Reclassification adjustment for net gains included in income, net of tax	0.1	0.1	17.7	0.2
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale, net of tax			7.2	0.2
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale, tax			1.8	(0.1)
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale, pre-tax			5.4	(0.1)
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, pre-tax	(0.4)	(0.9)	(0.8)	(23.9)
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	(0.1)	(0.2)	(0.2)	(5.9)
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, net of tax	(0.3)	(0.7)	(0.6)	(18.0)
Unrealized loss on derivatives, pre-tax	(22.1)	(15.0)	(12.1)	6.3
Unrealized loss on derivatives, tax expense (benefit)	(5.5)	(4.2)	(3.0)	1.1
Unrealized loss on derivatives, net of tax	(16.6)	(10.8)	(9.1)	5.2
Other comprehensive (loss) income, before tax	(79.9)	(177.4)	24.1	(431.4)
Other Comprehensive Income (Loss), tax expense (benefit)	(20.0)	(49.6)	6.0	(112.6)
Other Comprehensive Income (Loss), Net of Tax	$ (59.9)	$ (127.8)	$ 18.1	$ (318.8)