Investments in Marketable Debt Securities - Additional Information (Detail) - USD ($)	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
VRDOs
Schedule Of Available For Sale Securities [Line Items]
Marketable debt securities	$ 42,400,000		$ 7,400,000
Truck, Parts and Other
Schedule Of Available For Sale Securities [Line Items]
Gross realized gains from sales of marketable debt securities	400,000	$ 900,000
Gross realized loss from sales of marketable debt securities	200,000	$ 500,000
Other-than-temporary impairments recognized on investments	0		0
Marketable debt securities	$ 1,117,300,000		$ 1,020,400,000	[1]

[1] The December 31, 2018 consolidated balance sheet has been derived from audited financial statements.