Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows provided by operating activities:
Net income	$ 17,355	$ 16,724
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	42,689	40,415
Deferred income taxes	192	6,830
Tax deficiencies from share-based payment arrangement exercises		(148)
Share-based payment arrangement compensation expense	730	668
Distribution from affiliate	400
Equity in loss from affiliate	264	325
Changes in other current operating items:
Receivables	3,191	12,256
Prepaid expenses and other	1,672	1,012
Accounts payable and accrued liabilities	(670)	5,041
Insurance and claims accruals	2,371	1,165
Net cash provided by operating activities	65,220	80,151
Cash flows used for investing activities:
Other	(25)	(24)
Net cash used for investing activities	(46,965)	(39,678)
Cash flows used for financing activities:
Borrowings under credit facility and long-term debt	30,816	76,044
Repayment of borrowings under credit facility and long-term debt	(38,702)	(109,535)
Repurchase and retirement of common stock		(7,513)
Dividends on common stock	(1,635)	(1,622)
Issuance of common stock from share-based payment arrangement exercises	883	2,054
Employee taxes paid in exchange for shares withheld	(47)	(127)
Net cash used for financing activities	(8,685)	(40,699)
Net change in cash and cash equivalents	9,570	(226)
Cash and cash equivalents:
Beginning of period	488	434
End of period	10,058	208
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	3,156	6,532
Income taxes	7,892	(3,631)
Interest	29	127
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of assets	(2,974)	(4,137)
Cash flows used for investing activities:
Proceeds from revenue equipment dispositions	31,304	29,112
Revenue equipment additions	(76,210)	(66,009)
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Proceeds from revenue equipment dispositions		7
Revenue equipment additions	$ (2,034)	$ (2,764)