Consolidated condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Cash flows provided by operating activities:
Net income	$ 59,194	$ 39,424
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	53,008	51,227
Tires in service amortization	3,161	3,263
Gain on disposition of revenue equipment	(9,352)	(7,323)
Deferred income taxes	4,809	2,214
Share-based payment arrangement compensation expense	1,573	1,266
Changes in other current operating items:
Receivables	(37,619)	(7,596)
Prepaid expenses and other	(5,040)	(3,425)
Accounts payable	19,927	392
Insurance and claims accruals	3,675	1,843
Accrued and other current liabilities	5,334	2,080
Net cash provided by operating activities	98,670	83,365
Cash flows used for investing activities:
Other	(38)	(36)
Net cash used for investing activities	(35,004)	(61,438)
Cash flows used for financing activities:
Repurchase and retirement of common stock	(41,753)	0
Dividends on common stock	(9,843)	(6,626)
Issuance of common stock from share-based payment arrangement exercises, deferred compensation plan distributions and vesting of performance unit awards	919	497
Employee taxes paid in exchange for shares withheld	(1,610)	(1,253)
Net cash used for financing activities	(52,287)	(7,382)
Net change in cash and cash equivalents	11,379	14,545
Cash and cash equivalents:
Beginning of period	56,995	66,127
End of period	68,374	80,672
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	9,765	10,585
Supplemental disclosure of cash flow information:
Cash paid for income taxes	9,045	8,319
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of revenue equipment	(9,352)	(7,323)
Cash flows used for investing activities:
Revenue equipment additions	(54,202)	(98,877)
Proceeds from revenue equipment dispositions	23,854	38,995
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Revenue equipment additions	$ (4,618)	$ (1,520)