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Investments in Marketable Securities - Additional Information (Detail) - USD ($) | 9 Months Ended |
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Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items] |  |  |
Net unrealized gains (losses) on marketable equity securities recognized in investment income | $ (900,000) | $ 1,500,000 |
Allowance for credit losses | 0 |  | $ 0
Truck, Parts and Other |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Gross realized gains from sales of marketable debt securities | 2,700,000 | 1,400,000 |
Gross realized loss from sales of marketable debt securities | $ 1,800,000 | $ 3,400,000 |

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