Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows provided by operating activities:
Net income	$ 13,718	$ 13,546
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	25,427	22,543
Tires in service amortization	1,706	1,660
Gain on disposition of revenue equipment	(1,555)	(1,548)
Deferred income taxes	1,768	819
Share-based payment arrangement compensation expense	246	366
Equity in loss from affiliate	0	51
Changes in other current operating items:
Receivables	(782)	(315)
Prepaid expenses and other	930	918
Accounts payable	617	2,098
Insurance and claims accruals	2,695	1,278
Accrued and other current liabilities	(1,290)	(3,788)
Net cash provided by operating activities	43,480	37,628
Cash flows used for investing activities:
Other	(39)	(25)
Net cash used for investing activities	(36,632)	(6,736)
Cash flows used for financing activities:
Dividends on common stock	(2,193)	(1,637)
Repurchase and retirement of common stock	(597)	0
Issuance of common stock from share-based payment arrangement exercises	1,054	292
Employee taxes paid in exchange for shares withheld	(437)	(784)
Net cash used for financing activities	(2,173)	(2,129)
Net change in cash and cash equivalents	4,675	28,763
Cash and cash equivalents:
Beginning of period	31,461	56,763
End of period	36,136	85,526
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	11,107	5,030
Operating lease assets and liabilities acquired	88	1,135
Supplemental disclosure of cash flow information:
Income taxes	0	1,382
Interest	0	11
Revenue Equipment [Member]
Cash flows used for investing activities:
Revenue equipment additions	(44,086)	(13,159)
Proceeds from equipment dispositions	9,459	7,704
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Revenue equipment additions	$ (1,966)	$ (1,256)