Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows provided by operating activities:
Net income	$ 13,546	$ 10,331
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	22,543	21,815
Gain on disposition of assets	(1,548)	(1,211)
Deferred income taxes	819	1,050
Net noncurrent operating lease assets and liabilities	(537)
Share-based payment arrangement compensation expense	366	321
Equity in loss (earnings) from affiliate	51	(188)
Adoption of accounting standard		485
Changes in other current operating items:
Receivables	(315)	(7,535)
Prepaid expenses and other	1,899	624
Accounts payable	2,098	(3,571)
Insurance and claims accruals	1,278	(763)
Accrued and other current liabilities	(3,397)	1,824
Net cash provided by operating activities	36,803	23,182
Cash flows used for investing activities:
Other	(25)	(9)
Net cash used for investing activities	(5,911)	(27,392)
Cash flows used for financing activities:
Dividends on common stock	(1,637)	(1,365)
Issuance of common stock from share-based payment arrangement exercises	292	286
Employee taxes paid in exchange for shares withheld	(784)	(104)
Net cash used for financing activities	(2,129)	(1,183)
Net change in cash and cash equivalents	28,763	(5,393)
Beginning of period	56,763	15,791
End of period	85,526	10,398
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	5,030	12,493
Supplemental disclosure of cash flow information:
Income taxes	1,382	463
Interest	11	8
Revenue Equipment [Member]
Cash flows used for investing activities:
Revenue equipment additions	(13,159)	(37,696)
Proceeds from revenue equipment dispositions	8,529	14,924
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Revenue equipment additions	$ (1,256)	$ (4,611)