Other Comprehensive (Loss) Income And Accumulated Other Comprehensive Income - Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	$ 2,788.4	$ 2,505.2		$ (849.7)
Net Unrealized Losses on Fixed Maturities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(687.8)	(530.9)	$ (719.4)	505.8
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(165.8)	185.0	(1,215.1)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	8.9	3.5	(10.1)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(156.9)	188.5	(1,225.2)
Net Unrealized Losses on Investments with an Allowance for Credit Losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(3.2)	(2.5)	(2.2)	(3.7)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(0.3)	(0.3)	2.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.4)	0.0	(0.5)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.7)	(0.3)	1.5
Net Unrecognized Postretirement Benefit Income (Costs)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	8.4	9.5	(37.2)	52.1
OCI, before Reclassifications, Net of Tax, Attributable to Parent	1.4	(6.0)	15.2
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(2.5)	52.7	(0.3)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(1.1)	46.7	14.9
(Loss) Gain on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(2.2)	2.5	2.8	(1.9)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(5.4)	0.0	4.7
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.7	(0.3)	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(4.7)	(0.3)	4.7
Change in Discount Rate on Future Life Policyholder Benefits
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	380.3	160.6	241.1
OCI, before Reclassifications, Net of Tax, Attributable to Parent	219.7	(80.5)	1,090.8
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	219.7	(80.5)	1,090.8
Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(304.5)	(360.8)	(514.9)	$ (401.6)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	49.6	98.2	(102.4)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	6.7	55.9	(10.9)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 56.3	$ 154.1	$ (113.3)