Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows provided by operating activities:
Net income	$ 24,033	$ 17,355
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	44,008	42,689
Deferred income taxes	3,215	192
Share-based payment arrangement compensation expense	1,210	730
Distribution from affiliate	45	400
Equity in (earnings) loss from affiliate	(331)	264
Adoption of accounting standard (Note 2)	485
Changes in other current operating items:
Receivables	(8,854)	3,191
Prepaid expenses and other	(66)	1,672
Accounts payable and accrued liabilities	7,604	(670)
Insurance and claims accruals	(603)	2,371
Net cash provided by operating activities	67,375	65,220
Cash flows used for investing activities:
Other	(18)	(25)
Net cash used for investing activities	(59,177)	(46,965)
Cash flows used for financing activities:
Borrowings under credit facility and long-term debt		30,816
Repayment of borrowings under credit facility and long-term debt		(38,702)
Dividends on common stock	(2,731)	(1,635)
Issuance of common stock from share-based payment arrangement exercises	849	883
Employee taxes paid in exchange for shares withheld	(104)	(47)
Net cash used for financing activities	(1,986)	(8,685)
Net change in cash and cash equivalents	6,212	9,570
Cash and cash equivalents:
Beginning of period	15,791	488
End of period	22,003	10,058
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	(2,790)	3,156
Supplemental disclosure of cash flow information:
Income taxes	848	7,892
Interest	20	29
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of assets	(3,371)	(2,974)
Cash flows used for investing activities:
Revenue equipment additions	(85,325)	(76,210)
Proceeds from revenue equipment dispositions	31,954	31,304
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Revenue equipment additions	$ (5,788)	$ (2,034)