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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

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