Other Comprehensive Income and Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Stockholders' Equity Attributable to Parent	$ 2,589.8		$ 2,505.2
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax			9.5	$ (37.2)
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax			2.5	2.8
AOCI, Liability for Future Policy Benefit, Expected Future Policy Benefit, before Tax	160.6	$ 241.1
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Stockholders' Equity Attributable to Parent	(1.1)	(2.2)	(2.5)	(2.2)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	1.6	1.4
Reclassification from AOCI, Current Period, Tax	0.2	0.4
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.2)	(1.4)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.1	(0.4)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	1.4	0.0
AOCI Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	(335.0)	(454.7)	(360.8)	(514.9)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	17.0	62.2
Reclassification from AOCI, Current Period, Tax	(2.3)	0.4
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	8.8	(2.0)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(6.7)	(17.1)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	25.8	60.2
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	8.7	(37.6)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	0.0	0.0
Reclassification from AOCI, Current Period, Tax	0.1	(0.1)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.8)	(0.4)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.1	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.8)	(0.4)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Stockholders' Equity Attributable to Parent	2.5	2.7
OCI, before Reclassifications, Net of Tax, Attributable to Parent	0.0	0.0
Reclassification from AOCI, Current Period, Tax	0.0	0.1
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	(0.1)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.0	(0.1)
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent
Stockholders' Equity Attributable to Parent	(617.2)	(572.0)	$ (530.9)	$ (719.4)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(96.1)	147.5
Reclassification from AOCI, Current Period, Tax	(2.6)	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	9.8	(0.1)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	22.7	(39.8)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(86.3)	147.4
AOCI, Liability for Future Policy Benefit, Parent
Stockholders' Equity Attributable to Parent	272.1	154.4
OCI, before Reclassifications, Net of Tax, Attributable to Parent	111.5	(86.7)
Reclassification from AOCI, Current Period, Tax	0.0	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(29.6)	23.1
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 111.5	$ (86.7)