Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 60.0	$ 67.0	$ 118.4	$ 123.3
Other comprehensive income (loss), before tax:
Change in net unrealized gains (losses) during the period	2.5	(57.5)	(23.9)	20.7
Reclassification adjustment for net losses included in income	0.0	0.1	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	(0.1)	(0.4)	(0.3)	(0.8)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification and Tax	(5.1)	(23.6)	(22.5)	(14.1)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	4.6	1.5	8.1	2.0
Deferred tax (expense) benefit related to other comprehensive (loss) income	(0.5)	20.0	9.6	(6.0)
Other comprehensive income (loss), net of tax	1.4	(59.9)	(29.0)	18.1
Other comprehensive income (loss), before tax	1.9	(79.9)	(38.6)	24.1
Comprehensive income, net of tax	$ 61.4	$ 7.1	$ 89.4	$ 141.4