Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows provided by operating activities:
Net income	$ 49,896	$ 45,323
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	76,979	70,295
Tires in service amortization	4,937	5,065
Deferred income taxes	3,868	6,477
Share-based payment arrangement compensation expense	1,364	1,395
Equity in loss from affiliate	0	144
Changes in other current operating items:
Receivables	16,847	(1,418)
Prepaid expenses and other	(2,317)	(1,793)
Accounts payable	1,315	1,769
Insurance and claims accruals	4,900	2,544
Accrued and other current liabilities	4,535	(2,443)
Net cash provided by operating activities	154,707	122,021
Cash flows used for investing activities:
Other	(39)	(25)
Net cash used for investing activities	(93,748)	(98,489)
Cash flows used for financing activities:
Dividends on common stock	(7,705)	(40,448)
Repurchase and retirement of common stock	(597)	0
Issuance of common stock from share-based payment arrangement exercises	4,598	1,417
Employee taxes paid in exchange for shares withheld	(437)	(784)
Net cash used for financing activities	(4,141)	(39,815)
Net change in cash and cash equivalents	56,818	(16,283)
Cash and cash equivalents:
Beginning of period	31,461	56,763
End of period	88,279	40,480
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	2,355	10,527
Operating lease assets and liabilities acquired	88	1,664
Supplemental disclosure of cash flow information:
Income taxes	5,543	4,447
Interest	0	39
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of property, plant and equipment	(5,899)	(5,337)
Cash flows used for investing activities:
Property, plant and equipment additions	(138,457)	(127,156)
Proceeds from property, plant and equipment dispositions	46,872	32,114
Facility [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of property, plant and equipment	(1,718)	0
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Property, plant and equipment additions	(4,697)	(3,422)
Proceeds from property, plant and equipment dispositions	$ 2,573	$ 0