Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows provided by operating activities:
Net income	$ 39,424	$ 31,852
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	51,227	51,399
Tires in service amortization	3,263	3,328
Deferred income taxes	2,214	1,263
Share-based payment arrangement compensation expense	1,266	972
Changes in other current operating items:
Receivables	(7,596)	14,764
Prepaid expenses and other	(3,425)	(3,798)
Accounts payable	392	951
Insurance and claims accruals	1,843	4,105
Accrued and other current liabilities	2,080	3,002
Net cash provided by operating activities	83,365	104,067
Cash flows used for investing activities:
Other	(36)	(39)
Net cash used for investing activities	(61,438)	(59,190)
Cash flows used for financing activities:
Dividends on common stock	(6,626)	(4,398)
Repurchase and retirement of common stock	0	(597)
Issuance of common stock from share-based payment arrangement exercises	497	4,417
Employee taxes paid in exchange for shares withheld	(1,253)	(437)
Net cash used for financing activities	(7,382)	(1,015)
Net change in cash and cash equivalents	14,545	43,862
Cash and cash equivalents:
Beginning of period	66,127	31,461
End of period	80,672	75,323
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	10,585	(5,985)
Operating lease assets and liabilities acquired	0	88
Supplemental disclosure of cash flow information:
Income taxes	8,319	987
Interest	0	0
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of property, plant and equipment	(7,323)	(3,771)
Cash flows used for investing activities:
Property, plant and equipment additions	(98,877)	(75,463)
Proceeds from property, plant and equipment dispositions	38,995	19,788
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Property, plant and equipment additions	$ (1,520)	$ (3,476)