CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Statement of Comprehensive Income [Abstract]
Net income	$ 787.7	$ 722.4	$ 1,057.3
Other comprehensive income (loss), net:
Unrealized (loss) gain on AFS securities, net of tax effect of $0.9, $(41.4), and $225.3, respectively	(5.1)	116.9	(674.9)
Unrealized loss on SERP, net of tax effect of $0.0, $—, and $—, respectively	(0.1)	0.0	0.0
Unrealized (loss) gain on junior subordinated debt, net of tax effect of $0.5, $0.1, and $(1.2), respectively	(1.4)	(0.2)	3.7
Realized (gain) loss on sale of AFS securities included in income, net of tax effect of $4.4, $(10.2), and $1.9, respectively	(13.0)	30.2	(5.5)
Impairment of AFS securities included in income, net of tax effect of $0.4, $(0.4), and $— respectively	(1.2)	1.2	0.0
Net other comprehensive (loss) income	(20.8)	148.1	(676.7)
Comprehensive income	$ 766.9	$ 870.5	$ 380.6