Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows provided by operating activities:
Net income	$ 45,323	$ 39,290
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	70,295	66,280
Gain on disposition of assets	(5,337)	(5,206)
Deferred income taxes	6,477	7,070
Net noncurrent operating lease assets and liabilities	(463)	0
Share-based payment arrangement compensation expense	1,395	2,102
Distribution from affiliate	0	45
Equity in loss (earnings) from affiliate	144	(433)
Adoption of accounting standard	0	485
Changes in other current operating items:
Receivables	(1,418)	(8,103)
Prepaid expenses and other	1,348	994
Accounts payable	1,769	2,160
Insurance and claims accruals	2,544	(882)
Accrued and other current liabilities	(2,467)	8,295
Net cash provided by operating activities	119,610	112,097
Cash flows used for investing activities:
Other	(25)	(28)
Net cash used for investing activities	(96,078)	(88,842)
Cash flows used for financing activities:
Dividends on common stock	(40,448)	(4,098)
Issuance of common stock from share-based payment arrangement exercises	1,417	895
Employee taxes paid in exchange for shares withheld	(784)	(104)
Net cash used for financing activities	(39,815)	(3,307)
Net change in cash and cash equivalents	(16,283)	19,948
Beginning of period	56,763	15,791
End of period	40,480	35,739
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	10,527	(2,224)
Supplemental disclosure of cash flow information:
Income taxes	4,447	2,805
Interest	39	36
Revenue Equipment [Member]
Cash flows used for investing activities:
Revenue equipment additions	(127,156)	(130,856)
Proceeds from revenue equipment dispositions	34,525	48,440
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Revenue equipment additions	$ (3,422)	$ (6,398)