Investments in Marketable Debt Securities - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
VRDOs
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities	$ 49,700,000	$ 7,400,000
Truck, Parts and Other
Schedule Of Available For Sale Securities [Line Items]
Gross realized gains from sales of marketable debt securities	1,300,000	1,100,000	$ 1,400,000
Gross realized loss from sales of marketable debt securities	400,000	800,000	$ 500,000
Other-than-temporary impairments recognized on investments	0	0
Marketable debt securities	$ 1,162,100,000	$ 1,020,400,000

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Topic 320
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