Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Statement of Comprehensive Income [Abstract]
Net income	$ 67.0	$ 64.1	$ 123.3	$ 30.7
Other comprehensive income (loss), before tax:
Change in net unrealized (losses) gains during the period	(57.5)	(161.6)	20.7	(414.2)
Reclassification adjustment for net losses included in income	0.1	0.1	23.5	0.2
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	(0.4)	(0.9)	(0.8)	(23.9)
Change in unrealized (gain) loss on derivatives	(22.1)	(15.0)	(12.1)	6.3
Other comprehensive (loss) income, before tax	(79.9)	(177.4)	24.1	(431.4)
Deferred tax benefit (expense) related to other comprehensive (loss) income	20.0	49.6	(6.0)	112.6
Other comprehensive (loss) income, net of tax	(59.9)	(127.8)	18.1	(318.8)
Comprehensive income (loss), net of tax	$ 7.1	$ (63.7)	$ 141.4	$ (288.1)