Other Comprehensive (Loss) Income And Accumulated Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Net Unrealized Losses on Fixed Maturities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Current Period, Tax	$ (2.3)	$ (0.9)	$ 2.7
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	41.8	(50.3)	325.9
Net Unrealized Losses on Investments with an Allowance for Credit Losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Current Period, Tax	0.3	0.0	0.1
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.6	0.2	(0.4)
Net Unrecognized Postretirement Benefit Income (Costs)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Current Period, Tax	0.6	(13.8)	0.1
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.2	(12.5)	(4.0)
(Loss) Gain on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Current Period, Tax	(0.2)	(0.1)	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.7	(0.1)	(1.2)
Change in Discount Rate on Future Life Policyholder Benefits
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Current Period, Tax	0.0	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(58.3)	21.2	(289.9)
Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Current Period, Tax	(1.6)	(14.8)	2.9
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	$ (15.0)	$ (41.5)	$ 30.4