Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 226.0	$ 257.5	$ 202.2
Other comprehensive income (loss) before tax:
Change in net unrealized gain during period	48.6	144.8	(613.1)
Comprehensive income (loss), net of tax	260.6	378.1	(263.9)
Reclassification adjustment for net losses included in net income	0.0	23.5	24.4
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)
Change in unrealized losses on derivatives	(15.0)	(6.5)	(6.8)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	12.8	7.4	0.1
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)