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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	$ 3,005.5	$ 1,984.8	$ 3,005.5	$ 1,984.8	$ 1,986.6	$ 3,271.9	$ 1,970.9	$ 1,959.8
OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, after Tax					15.0
Net unrealized gain (loss) on derivatives	(12.3)	2.4	(7.1)	1.7
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
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)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(183.0)	(10.1)	(489.3)	(60.5)
Reclassification adjustment for net loss (gain) included in income	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 from Accumulated Other Comprehensive Income, Current Period, Net of Tax	18.7	(0.2)	18.9	(0.1)
Other Comprehensive Loss, Held-to-maturity Security, Reclassification Adjustment from AOCI for Noncredit Portion of OTTI, Sale, Tax			0.1	0.0
Other Comprehensive Loss, Held-to-maturity Security, Reclassification Adjustment from AOCI for Noncredit Portion of OTTI, Sale, after Tax			0.1	0.0
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before 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
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after 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
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)
Net unrealized loss on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	(499.1)		(499.1)		(29.0)
OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, after Tax			(4.4)
Net unrealized gain on derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized gain (loss) on derivatives			(4.2)		3.0
Net accumulated other comprehensive loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	$ (507.7)	$ 14.3	$ (507.7)	$ 14.3	$ (11.0)	$ (329.8)	$ 24.1	$ 56.6