Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$ 55,027,000	$ 90,284,000	$ 33,464,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	88,585,000	85,120,000	82,445,000
Gain on disposition of assets	(7,244,000)	(5,499,000)	(10,502,000)
Deferred income taxes	5,351,000	(47,228,000)	13,490,000
Share-based payment arrangement compensation expense	3,323,000	1,250,000	883,000
Distribution from affiliate	227,000	400,000
Equity in (earnings) loss from affiliate	(493,000)	271,000	1,018,000
Adoption of accounting standard (Note 2)	485,000
Tax benefits from share-based payment arrangement exercises			46,000
Changes in other current operating items:
Receivables	(8,131,000)	(5,974,000)	8,518,000
Prepaid expenses and other	(114,000)	(503,000)	(1,173,000)
Accounts payable	1,800,000	11,000	594,000
Insurance and claims accruals	1,926,000	6,737,000	3,205,000
Accrued liabilities	9,881,000	(2,990,000)	1,813,000
Net cash provided by operating activities	150,623,000	121,879,000	133,801,000
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Proceeds from revenue equipment dispositions	5,000	47,000	23,000
Proceeds from buildings and land, office equipment and other dispositions	5,000	47,000	23,000
Other	(15,000)	(43,000)	(42,000)
Net cash used for investing activities	(101,270,000)	(95,318,000)	(97,290,000)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Borrowings under credit facility and long-term debt		40,831,000	179,687,000
Repayment of borrowings under credit facility and long-term debt		(48,717,000)	(209,668,000)
Dividends on common stock	(5,459,000)	(4,361,000)	(3,252,000)
Repurchase and retirement of common stock	(3,756,000)		(7,513,000)
Issuance of common stock from share-based payment arrangement exercises	938,000	1,090,000	4,416,000
Employee taxes paid in exchange for shares withheld	(104,000)	(47,000)	(127,000)
Cash in lieu of fractional shares from stock split		(54,000)
Net cash used for financing activities	(8,381,000)	(11,258,000)	(36,457,000)
NET CHANGE IN CASH AND CASH EQUIVALENTS	40,972,000	15,303,000	54,000
CASH AND CASH EQUIVALENTS:
Beginning of year	15,791,000	488,000	434,000
End of year	56,763,000	15,791,000	488,000
Change in property and equipment not yet paid	(3,604,000)	(1,559,000)	4,511,000
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Income taxes	9,526,000	14,355,000	31,000
Interest	50,000	151,000	214,000
Revenue Equipment [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(161,160,000)	(148,856,000)	(154,984,000)
Proceeds from revenue equipment dispositions	67,262,000	61,227,000	65,082,000
Proceeds from buildings and land, office equipment and other dispositions	67,262,000	61,227,000	65,082,000
Buildings And Land Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	$ (7,362,000)	$ (7,693,000)	$ (7,369,000)