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Investments in Marketable Debt Securities - Additional Information (Detail) - USD ($)
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]      
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 600,000 $ 500,000  
Gross realized loss from sales of marketable debt securities 100,000 $ 200,000  
Allowance for credit losses $ 0   $ 0 [1]
[1] The December 31, 2020 consolidated balance sheet has been derived from audited financial statements.