Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$ 85,428	$ 69,500	$ 61,071
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	102,562	102,893	95,137
Tires in service amortization	6,433	6,575	6,861
Deferred income taxes	4,065	(924)	16,045
Share-based payment arrangement compensation expense	2,518	1,851	1,817
Equity in loss from affiliate	0	0	155
Changes in other current operating items:
Receivables	(16,745)	12,726	(13,638)
Prepaid expenses and other	(5,749)	(4,593)	(4,105)
Accounts payable	(823)	883	5,259
Insurance and claims accruals	2,419	7,866	3,626
Accrued and other current liabilities	7,391	3,272	(7,088)
Net cash provided by operating activities	171,204	189,598	156,460
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Disposition of investment in affiliate	0	0	1,168
Other	(37)	(39)	(25)
Net cash used for investing activities	(123,734)	(106,325)	(140,509)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Dividends on common stock	(54,735)	(52,366)	(42,089)
Issuance of common stock from share-based payment arrangement exercises, deferred compensation plan distributions and vesting of performance unit awards	876	4,793	1,620
Repurchase and retirement of common stock	0	(597)	0
Employee taxes paid in exchange for shares withheld	(2,743)	(437)	(784)
Net cash used for financing activities	(56,602)	(48,607)	(41,253)
NET CHANGE IN CASH AND CASH EQUIVALENTS	(9,132)	34,666	(25,302)
CASH AND CASH EQUIVALENTS:
Beginning of year	66,127	31,461	56,763
End of year	56,995	66,127	31,461
SUPPLEMENTAL NON-CASH DISCLOSURE:
Change in property and equipment not yet paid	(6,187)	3,230	249
Operating lease assets and liabilities acquired	0	88	1,668
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Income taxes	21,719	17,980	4,953
Interest	0	0	52
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of asset	(16,295)	(8,733)	(8,680)
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Property, plant and equipment additions	(193,892)	(166,395)	(184,680)
Proceeds from property, plant and equipment dispositions	75,596	64,185	47,745
Facility [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of asset	0	(1,718)	0
Buildings And Land Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Property, plant and equipment additions	(5,402)	(6,657)	(4,723)
Proceeds from property, plant and equipment dispositions	$ 1	$ 2,581	$ 6