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Investments in Marketable Debt Securities - Additional Information (Detail) - USD ($)
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
VRDOs      
Schedule Of Available For Sale Securities [Line Items]      
Marketable debt securities $ 0   $ 49,700,000
Truck, Parts and Other      
Schedule Of Available For Sale Securities [Line Items]      
Gross realized gains from sales of marketable debt securities 1,900,000 $ 1,000,000.0  
Gross realized loss from sales of marketable debt securities 300,000 $ 400,000  
Allowance for credit losses 0   0 [1]
Marketable debt securities $ 1,194,300,000   $ 1,162,100,000
[1] The December 31, 2019 consolidated balance sheet has been derived from audited financial statements.