Investments in Marketable Debt Securities - Additional Information (Detail) - Truck, Parts and Other - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Gross realized gains from sales of marketable debt securities	$ 1,100,000	$ 1,300,000
Gross realized loss from sales of marketable debt securities	700,000	$ 400,000
Other-than-temporary impairments recognized on investments	$ 0		$ 0

The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration