Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows provided by operating activities:
Net income	$ 39,290	$ 25,210
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	66,280	63,875
Deferred income taxes	7,070	1,992
Share-based payment arrangement compensation expense	2,102	916
Distribution from affiliate	45	400
Equity in (earnings) loss from affiliate	(433)	271
Adoption of accounting standard (Note 2)	485
Changes in other current operating items:
Receivables	(8,103)	(3,065)
Prepaid expenses and other	994	3,044
Accounts payable and accrued liabilities	10,455	1,192
Insurance and claims accruals	(882)	5,757
Net cash provided by operating activities	112,097	94,710
Cash flows used for investing activities:
Other	(28)	(34)
Net cash used for investing activities	(88,842)	(84,015)
Cash flows used for financing activities:
Borrowings under credit facility and long-term debt		30,816
Repayment of borrowings under credit facility and long-term debt		(38,702)
Dividends on common stock	(4,098)	(2,999)
Issuance of common stock from share-based payment arrangement exercises	895	962
Employee taxes paid in exchange for shares withheld	(104)	(47)
Cash in lieu of fractional shares from stock split		(54)
Net cash used for financing activities	(3,307)	(10,024)
Net change in cash and cash equivalents	19,948	671
Cash and cash equivalents:
Beginning of period	15,791	488
End of period	35,739	1,159
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	(2,224)	(3,244)
Supplemental disclosure of cash flow information:
Income taxes	2,805	11,031
Interest	36	41
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of assets	(5,206)	(4,882)
Cash flows used for investing activities:
Revenue equipment additions	(130,856)	(135,010)
Proceeds from revenue equipment dispositions	48,440	53,586
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Revenue equipment additions	$ (6,398)	$ (2,557)