Consolidated Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of equipment and facility	$ (4,843)	$ (3,360)
Cash flows used for investing activities:
Properties and equipment additions	(139,788)	(102,752)
Proceeds from properties and equipment dispositions	43,451	$ 35,429
Facility [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of equipment and facility	(3,712)
Buildings And Land Office Equipment And Other [Member]
Cash flows used for investing activities:
Properties and equipment additions	(11,784)	$ (27,870)
Proceeds from properties and equipment dispositions	4,625	17
Net income	26,955	20,865
Depreciation	55,469	50,489
Deferred income taxes	(383)	(3,795)
Tax benefits from share-based payment arrangement exercises	451	141
Excess tax benefits from share-based payment arrangement exercises	(432)	(97)
Share-based payment arrangement compensation expense	1,189	720
Equity in earnings from affiliate	184	(453)
Receivables	15,300	(3,825)
Prepaid expenses and other	1,057	1,793
Accounts payable and accrued liabilities	9,353	(3,450)
Insurance and claims accruals	834	(645)
Net cash provided by operating activities	101,422	58,383
Other	(30)	(27)
Net cash used for investing activities	(103,526)	(95,203)
Cash flows provided by financing activities:
Borrowings under credit facility and long-term debt	94,437	102,912
Repayment of borrowings under credit facility and long-term debt	(92,530)	(78,231)
Dividends on common stock	(2,518)	(2,504)
Issuance of common stock from share-based payment arrangement exercises	2,363	1,055
Excess tax benefits from share-based payment arrangement exercises	432	97
Change in checks issued in excess of cash balances	(58)	28
Net cash provided by financing activities	2,126	23,357
Net change in cash and cash equivalents	22	(13,463)
Cash and cash equivalents:
Beginning of period	123	13,650
End of period	145	187
Supplemental non-cash disclosure:
Change in property and equipment not yet paid	10,237	4,264
Supplemental disclosure of cash flow information:
Income taxes	(4,451)	20,001
Interest	$ 136	$ 114