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Investments in Marketable Debt Securities - Additional Information (Detail) - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
VRDOs      
Schedule Of Available For Sale Securities [Line Items]      
Marketable debt securities $ 49,800,000   $ 7,400,000
Truck, Parts and Other      
Schedule Of Available For Sale Securities [Line Items]      
Gross realized gains from sales of marketable debt securities 1,000,000.0 $ 1,100,000  
Gross realized loss from sales of marketable debt securities 400,000 $ 700,000  
Other-than-temporary impairments recognized on investments 0   0
Marketable debt securities $ 1,107,800,000   $ 1,020,400,000 [1]
[1] The December 31, 2018 consolidated balance sheet has been derived from audited financial statements.