Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
OPERATING ACTIVITIES:
Net income	$ 7,900,000	$ 23,994,000	$ 38,899,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	55,107,000	49,387,000	42,274,000
Bad debt expense	728,000	889,000	341,000
Stock compensation—net of excess tax benefits	912,000	732,000	614,000
Provision for (benefit from) deferred income taxes	1,625,000	7,206,000	(24,630,000)
Reclassification of other than temporary impairment in marketable equity securities			42,000
Recognized (gain) loss on marketable equity securities	(4,753,000)	5,388,000	(4,735,000)
(Loss) gain on sale or disposal of equipment	583,000	(1,306,000)	(58,000)
Changes in operating assets and liabilities:
Accounts receivable	884,000	(5,970,000)	(1,298,000)
Prepaid expenses, deposits, inventories, and other assets	1,865,000	(137,000)	(1,095,000)
Income taxes refundable	1,372,000	(76,000)	(155,000)
Trade accounts payable	(2,233,000)	1,731,000	682,000
Accrued expenses and other liabilities	20,307,000	509,000	(266,000)
Net cash provided by operating activities	84,297,000	82,347,000	50,614,000
INVESTING ACTIVITIES:
Purchases of property and equipment	(79,354,000)	(73,882,000)	(67,674,000)
Proceeds from disposition of equipment	14,263,000	24,904,000	18,766,000
Sales of marketable equity securities	2,984,000	1,045,000	6,833,000
Purchases of marketable equity securities, net of return of capital	(203,000)	(7,318,000)	(3,211,000)
Net cash used in investing activities	(62,310,000)	(55,251,000)	(45,286,000)
FINANCING ACTIVITIES:
Borrowings under line of credit	590,902,000	615,612,000	483,297,000
Repayments under line of credit	(584,047,000)	(605,419,000)	(485,163,000)
Borrowings of long-term debt	60,203,000	52,717,000	55,415,000
Repayments of long-term debt	(70,917,000)	(82,442,000)	(48,110,000)
Borrowings under margin account	527,000	7,584,000	3,412,000
Repayments under margin account	(4,334,000)	(2,206,000)	(7,867,000)
Repurchases of common stock	(14,285,000)	(13,369,000)	(6,348,000)
Proceeds from exercise of stock options	0	485,000	123,000
Net cash used in financing activities	(21,951,000)	(27,038,000)	(5,241,000)
NET INCREASE IN CASH AND CASH EQUIVALENTS	36,000	58,000	87,000
CASH AND CASH EQUIVALENTS—Beginning of year	282,000	224,000	137,000
CASH AND CASH EQUIVALENTS—End of year	318,000	282,000	224,000
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION—
Interest	8,612,000	6,095,000	3,905,000
Income taxes	671,000	217,000	518,000
NONCASH INVESTING AND FINANCING ACTIVITIES—
Purchases of revenue equipment included in accounts payable	$ 366,000	$ 1,597,000	$ 2,973,000