Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$ 70,373	$ 110,354	$ 85,428
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	116,722	111,014	102,562
Tires in service amortization	7,172	6,604	6,433
Deferred income taxes	(14,579)	11,878	4,065
Share-based payment arrangement compensation expense	2,319	2,814	2,518
us-gaap_IncreaseDecreaseInOperatingCapitalAbstract
Receivables	12,751	(23,547)	(16,745)
Prepaid expenses and other	(3,939)	(8,227)	(5,749)
Accounts payable	(3,615)	11,893	(823)
Insurance and claims accruals	1,270	3,733	2,419
Accrued and other current liabilities	(10,484)	6,352	7,391
Net cash provided by operating activities	164,378	219,489	171,204
CASH FLOW USED FOR INVESTING ACTIVITIES:
Other	(45)	(38)	(37)
Net cash used for investing activities	(172,540)	(134,958)	(123,734)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Dividends on common stock	(19,509)	(19,563)	(54,735)
Repurchase and retirement of common stock	0	(41,753)	0
Issuance of common stock from share-based payment arrangement exercises, deferred compensation plan distributions and vesting of performance unit awards	1,210	2,000	876
Employee taxes paid in exchange for shares withheld	(926)	(1,610)	(2,743)
Net cash used for financing activities	(19,225)	(60,926)	(56,602)
NET CHANGE IN CASH AND CASH EQUIVALENTS	(27,387)	23,605	(9,132)
CASH AND CASH EQUIVALENTS:
Beginning of year	80,600	56,995	66,127
End of year	53,213	80,600	56,995
SUPPLEMENTAL NON-CASH DISCLOSURE:
Change in property and equipment not yet paid	(1,612)	10,470	(6,187)
Operating lease assets and liabilities acquired	89	318	0
Income taxes	36,978	23,649	21,719
Interest	10	65	0
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of revenue equipment	(13,612)	(13,379)	(16,295)
CASH FLOW USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(231,943)	(162,556)	(193,892)
Proceeds from revenue equipment dispositions	68,009	41,695	75,596
Buildings And Land Office Equipment And Other [Member]
CASH FLOW USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(8,614)	(14,067)	(5,402)
Proceeds from revenue equipment dispositions	$ 53	$ 8	$ 1