Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income	$ 72.7	$ 85.7	$ 196.0	$ 116.4
Other comprehensive income (loss), before tax:
Change in net unrealized losses during the period	(72.1)	(236.4)	(51.4)	(650.6)
Reclassification adjustment for net losses included in income	0.0	24.2	23.5	24.4
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	0.0	0.0	(7.2)	0.2
OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax	(0.3)	(1.7)	(1.1)	(25.6)
Change in unrealized gain on derivatives	(11.0)	(15.9)	(23.1)	(9.6)
Other comprehensive loss, before tax	(83.4)	(229.8)	(59.3)	(661.2)
Deferred tax benefit related to other comprehensive loss	20.7	51.9	14.7	164.5
Other comprehensive loss, net of tax	(62.7)	(177.9)	(44.6)	(496.7)
Comprehensive income (loss), net of tax	$ 10.0	$ (92.2)	$ 151.4	$ (380.3)