Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$ 110,354	$ 85,428	$ 69,500
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	111,014	102,562	102,893
Tires in service amortization	6,604	6,433	6,575
Deferred income taxes	11,878	4,065	(924)
Share-based payment arrangement compensation expense	2,814	2,518	1,851
Changes in other current operating items:
Receivables	(23,547)	(16,745)	12,726
Prepaid expenses and other	(8,227)	(5,749)	(4,593)
Accounts payable	11,893	(823)	883
Insurance and claims accruals	3,733	2,419	7,866
Accrued and other current liabilities	6,352	7,391	3,272
Net cash provided by operating activities	219,489	171,204	189,598
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Other	(38)	(37)	(39)
Net cash used for investing activities	(134,958)	(123,734)	(106,325)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Repurchase and retirement of common stock	(41,753)	0	(597)
Dividends on common stock	(19,563)	(54,735)	(52,366)
Issuance of common stock from share-based payment arrangement exercises, deferred compensation plan distributions and vesting of performance unit awards	2,000	876	4,793
Employee taxes paid in exchange for shares withheld	(1,610)	(2,743)	(437)
Net cash used for financing activities	(60,926)	(56,602)	(48,607)
NET CHANGE IN CASH AND CASH EQUIVALENTS	23,605	(9,132)	34,666
CASH AND CASH EQUIVALENTS:
Beginning of year	56,995	66,127	31,461
End of year	80,600	56,995	66,127
SUPPLEMENTAL NON-CASH DISCLOSURE:
Change in property and equipment not yet paid	10,470	(6,187)	3,230
Operating lease assets and liabilities acquired	318	0	88
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Income taxes	23,649	21,719	17,980
Interest	65	0	0
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of asset	(13,379)	(16,295)	(8,733)
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(162,556)	(193,892)	(166,395)
Proceeds from revenue equipment dispositions	41,695	75,596	64,185
Facility [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of asset	0	0	(1,718)
Buildings And Land Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(14,067)	(5,402)	(6,657)
Proceeds from revenue equipment dispositions	$ 8	$ 1	$ 2,581