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Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Pretax total |  | $ (82.9) | $ (142.5) | $ 14.7
Unrealized Gains (Losses) on Debt Securities |  |  |  |
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Net realized (losses) gains on sales of debt securities |  | (80.9) | (136.5) | 20.3
Credit losses recognized on debt securities |  | 0.0 | 0.0 | (0.1)
Pretax total |  | (80.9) | (136.5) | 20.2
Tax effect |  | 19.8 | 36.2 | (5.0)
Pension Benefit Adjustment |  |  |  |
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Amortization of net actuarial loss | [1] | (2.0) | (5.8) | (6.8)
Amortization of prior service (cost) credit | [1] | 0.0 | (0.2) | 1.3
Pretax total | [1] | (2.0) | (6.0) | (5.5)
Tax effect | [1] | $ 0.5 | $ 1.6 | $ 1.5
[1] | Amounts are components of net periodic cost. See Note 16 Employee Benefit Plans for additional details.

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