Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 85.7	$ 47.1	$ 116.4	$ 141.0
Other comprehensive loss, before tax:
Change in net unrealized loss during period	(236.4)	(13.4)	(650.6)	(81.0)
Reclassification adjustment for net loss (gain) included in income	24.2	(0.3)	24.4	(0.2)
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	0.0	0.0	0.2	0.0
OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax	(1.7)	(2.5)	(25.6)	22.3
Change in unrealized (gain) loss on derivatives	(15.9)	3.2	(9.6)	2.3
Other comprehensive loss, before tax	(229.8)	(13.0)	(661.2)	(56.6)
Deferred tax benefit related to other comprehensive loss	51.9	3.2	164.5	14.3
Other comprehensive loss, net of tax	(177.9)	(9.8)	(496.7)	(42.3)
Comprehensive (loss) income, net of tax	$ (92.2)	$ 37.3	$ (380.3)	$ 98.7