Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows provided by operating activities:
Net income	$ 28,736,000	$ 24,033,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	46,005,000	44,008,000
Gain on disposition of assets	(2,778,000)	(3,371,000)
Deferred income taxes	2,601,000	3,215,000
Net noncurrent operating lease assets and liabilities	(553,000)	0
Share-based payment arrangement compensation expense	1,002,000	1,210,000
Distribution from affiliate	0	45,000
Equity in loss (earnings) from affiliate	68,000	(331,000)
Adoption of accounting standard	0	485,000
Changes in other current operating items:
Receivables	(2,798,000)	(8,854,000)
Prepaid expenses and other	200,000	(66,000)
Accounts payable	87,000	2,683,000
Insurance and claims accruals	2,804,000	(603,000)
Accrued and other current liabilities	(1,865,000)	4,921,000
Net cash provided by operating activities	73,509,000	67,375,000
Cash flows used for investing activities:
Other	(25,000)	(18,000)
Net cash used for investing activities	(44,178,000)	(59,177,000)
Cash flows used for financing activities:
Dividends on common stock	(3,276,000)	(2,731,000)
Issuance of common stock from share-based payment arrangement exercises	758,000	849,000
Employee taxes paid in exchange for shares withheld	(784,000)	(104,000)
Net cash used for financing activities	(3,302,000)	(1,986,000)
Net change in cash and cash equivalents	26,029,000	6,212,000
Beginning of period	56,763,000	15,791,000
End of period	82,792,000	22,003,000
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	11,418,000	(2,790,000)
Supplemental disclosure of cash flow information:
Income taxes	4,143,000	848,000
Interest	23,000	20,000
Revenue Equipment [Member]
Cash flows used for investing activities:
Revenue equipment additions	(60,784,000)	(85,325,000)
Proceeds from revenue equipment dispositions	18,879,000	31,954,000
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Revenue equipment additions	$ (2,248,000)	$ (5,788,000)