Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows provided by operating activities:
Net income	$ 10,331,000	$ 8,214,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	21,815,000	21,383,000
Deferred income taxes	1,050,000	3,416,000
Share-based payment arrangement compensation expense	321,000	149,000
Distribution from affiliate		400,000
Equity in (earnings) loss from affiliate	(188,000)	143,000
Adoption of accounting standard (Note 2)	485,000
Changes in other current operating items:
Receivables	(7,535,000)	771,000
Prepaid expenses and other	624,000	2,205,000
Accounts payable and accrued liabilities	(1,747,000)	954,000
Insurance and claims accruals	(763,000)	1,142,000
Net cash provided by operating activities	23,182,000	37,674,000
Cash flows used for investing activities:
Other	(9,000)	(16,000)
Net cash used for investing activities	(27,392,000)	(20,962,000)
Cash flows used for financing activities:
Borrowings under credit facility and long-term debt		30,816,000
Repayment of borrowings under credit facility and long-term debt		(38,702,000)
Dividends on common stock	(1,365,000)	(817,000)
Issuance of common stock from share-based payment arrangement exercises	286,000	615,000
Employee taxes paid in exchange for shares withheld	(104,000)	(47,000)
Net cash used for financing activities	(1,183,000)	(8,135,000)
Net change in cash and cash equivalents	(5,393,000)	8,577,000
Cash and cash equivalents:
Beginning of period	15,791,000	488,000
End of period	10,398,000	9,065,000
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	12,493,000	1,139,000
Supplemental disclosure of cash flow information:
Income taxes	463,000	290,000
Interest	8,000	24,000
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of assets	(1,211,000)	(1,103,000)
Cash flows used for investing activities:
Revenue equipment additions	(37,696,000)	(32,797,000)
Proceeds from revenue equipment dispositions	14,924,000	12,594,000
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Revenue equipment additions	$ (4,611,000)	$ (743,000)