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Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] |  |  |  |  |
Amortization of net actuarial loss |  |  |  | $ (1.7) |
Unrealized Gains (Losses) on Debt Securities |  |  |  |  |
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] |  |  |  |  |
Net realized losses on sales of debt securities |  | $ (326.6) | $ (5.1) | (331.0) | $ (12.4)
Tax effect |  | 84.6 | 1.2 | 85.7 | 3.1
Pension Benefit Adjustment |  |  |  |  |
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] |  |  |  |  |
Amortization of net actuarial loss | [1] | (0.6) | (0.4) | (1.7) | (1.5)
Tax effect | [1] | $ 0.1 | $ 0.0 | $ 0.4 | $ 0.4
[1] | Amounts are components of net periodic cost. See Note 12 Employee Benefit Plans for additional details.

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