Investments in Marketable Debt Securities - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Allowance for credit losses	$ 0	$ 0
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	2,700,000	1,300,000	$ 1,100,000
Gross realized loss from sales of marketable debt securities	300,000	400,000	$ 800,000
Allowance for credit losses	0	0
Marketable debt securities	$ 1,429,000,000.0	$ 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
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

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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
-Name Accounting Standards Codification
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-Section 50
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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