Investments in Marketable Securities - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Allowance for credit losses	$ 0	$ 0		$ 0
Truck, Parts and Other
Schedule Of Available For Sale Securities [Line Items]
Gross realized gains from sales of marketable debt securities		1,600,000	$ 1,900,000
Gross realized loss from sales of marketable debt securities		100,000	$ 300,000
Realized gains (losses) on marketable equity securities	$ 0	$ 0

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).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Paragraph 9
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Amount of realized loss 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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Amount of realized gain (loss) from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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