Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Detail) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||||
|---|---|---|---|---|---|---|
Jun. 30, 2026 |
Jun. 30, 2025 |
Jun. 30, 2026 |
Jun. 30, 2025 |
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| Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] | ||||||
| Amortization of net actuarial loss | $ (1.0) | |||||
| Unrealized Gains (Losses) on Debt Securities | ||||||
| Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] | ||||||
| Net realized losses on sales of debt securities | $ (1.5) | $ (5.4) | (2.8) | $ (6.9) | ||
| Tax effect | 0.4 | 1.4 | 0.7 | 1.8 | ||
| Pension Benefit Adjustment | ||||||
| Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] | ||||||
| Amortization of net actuarial loss | [1] | (0.5) | (0.4) | (1.0) | (0.9) | |
| Tax effect | [1] | $ 0.2 | $ 0.1 | $ 0.3 | $ 0.2 | |
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