Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 55.5	$ 72.7	$ 173.9	$ 196.0
Other comprehensive income (loss), before tax:
Change in net unrealized gains (losses) during the period	158.5	(72.1)	134.6	(51.4)
Reclassification adjustment for net losses included in income	0.0	0.0	0.0	23.5
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	0.0	0.0	0.0	(7.2)
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax	(0.2)	(0.3)	(0.5)	(1.1)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification and Tax	12.4	(13.1)	(10.1)	(27.2)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	3.1	2.1	11.2	4.1
Deferred tax (expense) benefit related to other comprehensive (loss) income	(43.2)	20.7	(33.6)	14.7
Other comprehensive income (loss), net of tax	130.6	(62.7)	101.6	(44.6)
Other comprehensive income (loss), before tax	173.8	(83.4)	135.2	(59.3)
Comprehensive income, net of tax	$ 186.1	$ 10.0	$ 275.5	$ 151.4