Investments in Marketable Debt Securities - Additional Information (Detail) - USD ($)	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
VRDOs
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities	$ 32,800,000		$ 49,700,000
Truck, Parts and Other
Schedule Of Available For Sale Securities [Line Items]
Gross realized gains from sales of marketable debt securities	500,000	$ 200,000
Gross realized loss from sales of marketable debt securities	200,000	$ 100,000
Allowance for credit losses	0		0
Marketable debt securities	$ 1,097,000,000.0		$ 1,162,100,000

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
-Section 45
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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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