Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$ 61,071	$ 55,027	$ 90,284
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	95,137	88,585	85,120
Gain on disposition of revenue equipment	(8,680)	(7,244)	(5,499)
Deferred income taxes	16,045	5,351	(47,228)
Share-based payment arrangement compensation expense	1,817	3,323	1,250
Equity in loss (earnings) from affiliate	155	(493)	271
Distribution from affiliate	0	227	400
Adoption of accounting standard	0	485	0
Changes in other current operating items:
Receivables	(13,638)	(8,131)	(5,974)
Prepaid expenses and other	(524)	(114)	(503)
Accounts payable	5,259	1,800	11
Insurance and claims accruals	3,626	1,926	6,737
Accrued and other current liabilities	(7,088)	9,881	(2,990)
Net cash provided by operating activities	153,180	150,623	121,879
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Disposition of investment in affiliate	1,168	0	0
Other	(25)	(15)	(43)
Net cash used for investing activities	(137,229)	(101,270)	(95,318)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Dividends on common stock	(42,089)	(5,459)	(4,361)
Borrowings under credit facility and long-term debt	0	0	40,831
Repayment of borrowings under credit facility and long-term debt	0	0	(48,717)
Repurchase and retirement of common stock	0	(3,756)	0
Issuance of common stock from share-based payment arrangement exercises	1,620	938	1,090
Employee taxes paid in exchange for shares withheld	(784)	(104)	(47)
Cash in lieu of fractional shares from stock split	0	0	(54)
Net cash used for financing activities	(41,253)	(8,381)	(11,258)
NET CHANGE IN CASH AND CASH EQUIVALENTS	(25,302)	40,972	15,303
CASH AND CASH EQUIVALENTS:
Beginning of year	56,763	15,791	488
End of year	31,461	56,763	15,791
SUPPLEMENTAL NON-CASH DISCLOSURE:
Change in property and equipment not yet paid	249	(3,604)	(1,559)
Operating lease assets and liabilities acquired	1,668	0	0
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:
Income taxes	4,953	9,526	14,355
Interest	52	50	151
Revenue Equipment [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(184,680)	(161,160)	(148,856)
Proceeds from equipment dispositions	51,025	67,262	61,227
Buildings And Land Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(4,723)	(7,362)	(7,693)
Proceeds from equipment dispositions	$ 6	$ 5	$ 47