Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$ 69,500	$ 61,071	$ 55,027
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	102,893	95,137	88,585
Tires in service amortization	6,575	6,861	6,982
Deferred income taxes	(924)	16,045	5,351
Share-based payment arrangement compensation expense	1,851	1,817	3,323
Equity in loss (earnings) from affiliate	0	155	(493)
Distribution from affiliate	0	0	227
Adoption of accounting standard	0	0	485
Changes in other current operating items:
Receivables	12,726	(13,638)	(8,131)
Prepaid expenses and other	(4,593)	(4,105)	(3,515)
Accounts payable	883	5,259	1,800
Insurance and claims accruals	7,866	3,626	1,926
Accrued and other current liabilities	3,272	(7,088)	9,881
Net cash provided by operating activities	189,598	156,460	154,204
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Disposition of investment in affiliate	0	1,168	0
Other	(39)	(25)	(15)
Net cash used for investing activities	(106,325)	(140,509)	(104,851)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Dividends on common stock	(52,366)	(42,089)	(5,459)
Issuance of common stock from share-based payment arrangement exercises	4,793	1,620	938
Repurchase and retirement of common stock	(597)	0	(3,756)
Employee taxes paid in exchange for shares withheld	(437)	(784)	(104)
Net cash used for financing activities	(48,607)	(41,253)	(8,381)
NET CHANGE IN CASH AND CASH EQUIVALENTS	34,666	(25,302)	40,972
CASH AND CASH EQUIVALENTS:
Beginning of year	31,461	56,763	15,791
End of year	66,127	31,461	56,763
SUPPLEMENTAL NON-CASH DISCLOSURE:
Change in property and equipment not yet paid	3,230	249	(3,604)
Operating lease assets and liabilities acquired	88	1,668	0
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Income taxes	17,980	4,953	9,526
Interest	0	52	50
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of property, plant and equipment	(8,733)	(8,680)	(7,244)
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Property, plant and equipment additions	(166,395)	(184,680)	(161,160)
Proceeds from property, plant and equipment dispositions	64,185	47,745	63,681
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	0
Buildings And Land Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Property, plant and equipment additions	(6,657)	(4,723)	(7,362)
Proceeds from property, plant and equipment dispositions	$ 2,581	$ 6	$ 5