Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Detail) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||||
|---|---|---|---|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
|||
| Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] | ||||||
| Amortization of net actuarial loss | $ (1.5) | |||||
| Unrealized Gains (Losses) on Debt Securities | ||||||
| Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] | ||||||
| Net realized losses on sales of debt securities | $ (1.4) | $ (2.0) | (4.2) | $ (8.9) | ||
| Tax effect | 0.4 | 0.4 | 1.1 | 2.2 | ||
| Pension Benefit Adjustment | ||||||
| Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] | ||||||
| Amortization of net actuarial loss | [1] | (0.5) | (0.5) | (1.5) | (1.4) | |
| Tax effect | [1] | $ 0.1 | $ 0.2 | $ 0.4 | $ 0.4 | |
| ||||||