Other Comprehensive Income - Schedule of Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Equity [Abstract]
Other comprehensive income (loss), before tax	$ 45.9	$ 160.7	$ (621.3)
Deferred tax (expense) benefit related to other comprehensive (loss) income	(11.3)	(40.1)	155.2
Other Comprehensive Income (Loss), Net of Tax	34.6	120.6	(466.1)
Before Tax Amount
Change in net unrealized gain during period	48.6	144.8	(613.1)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(17.7)	(18.9)
Reclassification adjustment for net losses included in net income	0.0	23.5	24.4
OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, Tax		(1.8)	0.1
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	0.0	(7.2)	0.2
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax	(0.5)	(1.3)	(26.1)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, Tax	(4.0)	(1.5)	(1.8)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, after Tax	(11.0)	(5.0)	(5.0)
Change in unrealized losses on derivatives	(15.0)	(6.5)	(6.8)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	9.5	5.6	0.1
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	12.8	7.4	0.1
Tax (Expense) Benefit
Change in net unrealized gain during period	(12.0)	(36.1)	152.4
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), Tax	0.0	0.3	6.6
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	3.3	1.8	0.0
Reclassification adjustment for net losses included in net income		(5.8)	(5.5)
Net of Tax Amount
Change in net unrealized losses during period	36.6	108.7	(460.7)
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after Tax	(0.5)	(1.0)	(19.5)
Stockholders' Equity Attributable to Parent	$ 3,304.0	3,227.5	3,073.8	$ 1,986.6
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Net of Tax		$ (5.4)	$ 0.1