Other Comprehensive Income/Loss - Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	$ 3,085.5	$ 3,005.5	$ 3,085.5	$ 3,005.5	$ 3,073.8	$ 3,121.1	$ 3,271.9	$ 1,986.6
Net unrealized gain (loss) on derivatives	(8.3)	(12.3)	(17.4)	(7.1)
Change in net unrealized gain during period, pre-tax	(72.1)	(236.4)	(51.4)	(650.6)
Change in net unrealized gain during period, tax expense (benefit)	(17.9)	(53.4)	(12.7)	(161.3)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(54.2)	(183.0)	(38.7)	(489.3)
Reclassification adjustment for net losses included in income	0.0	24.2	23.5	24.4
Reclassification adjustment for net gains included in income, tax expense (benefit)	0.0	5.5	5.8	5.5
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	18.7	17.7	18.9
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax	(0.3)	(1.7)	(1.1)	(25.6)
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	(0.1)	(0.4)	(0.3)	(6.3)
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after Tax	(0.2)	(1.3)	(0.8)	(19.3)
Unrealized loss on derivatives, pre-tax	(11.0)	(15.9)	(23.1)	(9.6)
Unrealized loss on derivatives, tax expense (benefit)	(2.7)	(3.6)	(5.7)	(2.5)
Other comprehensive loss, before tax	(83.4)	(229.8)	(59.3)	(661.2)
Other Comprehensive Income (Loss), tax expense (benefit)	(20.7)	(51.9)	(14.7)	(164.5)
Other Comprehensive Income (Loss), Net of Tax	(62.7)	(177.9)	(44.6)	(496.7)
Net unrealized loss on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	(497.4)		(497.4)		(471.0)
OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, after Tax			(5.4)		(4.5)
Net unrealized loss on derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized gain (loss) on derivatives			(18.9)		(1.6)
Net accumulated other comprehensive loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	$ (521.7)	$ (507.7)	$ (521.7)	$ (507.7)	$ (477.1)	$ (459.0)	$ (329.8)	$ (11.0)