Consolidate Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of revenue equipment	$ (5,584,000)	$ (4,387,000)	$ (5,972,000)
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(174,165,000)	(131,645,000)	(102,808,000)
Proceeds from revenue equipment dispositions	56,011,000	$ 43,933,000	$ 48,048,000
Facility [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of revenue equipment	(4,104,000)
Buildings And Land Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(13,482,000)	$ (30,885,000)	$ (18,031,000)
Proceeds from revenue equipment dispositions	6,594,000	28,000	28,000
Net income	35,745,000	29,834,000	30,147,000
Depreciation	75,323,000	68,243,000	64,476,000
Deferred income taxes	14,720,000	9,422,000	4,303,000
Tax benefits from share-based payment arrangement exercises	373,000	159,000	189,000
Excess tax benefits from share-based payment arrangement exercises	(432,000)	(115,000)	(165,000)
Allocated Share-based Compensation Expense	1,404,000	921,000	1,084,000
Equity in loss (earnings) from affiliate	$ 296,000	$ (246,000)	(111,000)
Income before income taxes attributable to noncontrolling interest			84,000
Receivables	$ 7,968,000	$ (14,913,000)	(6,478,000)
Prepaid expenses and other	(1,274,000)	(1,586,000)	(25,000)
Accounts payable	(4,853,000)	(723,000)	2,387,000
Insurance and claims accruals	2,237,000	(406,000)	566,000
Accrued liabilities	6,419,000	(4,232,000)	(1,298,000)
Net cash provided by operating activities	$ 128,238,000	$ 81,971,000	89,187,000
Decrease in cash and cash equivalents resulting from change to equity method of accounting			(1,924,000)
Other	$ (39,000)	$ (37,000)	(35,000)
Net cash used for investing activities	(125,081,000)	(118,606,000)	(74,722,000)
CASH FLOWS (USED FOR) PROVIDED BY FINANCING ACTIVITIES:
Borrowings under credit facility and long-term debt	141,908,000	168,444,000	82,697,000
Repayment of borrowings under credit facility and long-term debt	(128,414,000)	$ (144,071,000)	$ (85,423,000)
Repurchase and retirement of common stock	(16,175,000)
Dividends on common stock	(3,342,000)	$ (3,339,000)	$ (2,769,000)
Issuance of common stock from share-based payment arrangement exercises	3,490,000	1,214,000	$ 1,126,000
Change in checks issued in excess of cash balances	(745,000)	745,000
Excess tax benefits from share-based payment arrangement exercises	$ 432,000	$ 115,000	$ 165,000
Noncontrolling interest distributions			(84,000)
Net cash (used for) provided by financing activities	$ (2,846,000)	$ 23,108,000	(4,288,000)
NET CHANGE IN CASH AND CASH EQUIVALENTS	311,000	(13,527,000)	10,177,000
CASH AND CASH EQUIVALENTS:
Beginning of year	123,000	13,650,000	3,473,000
End of year	434,000	123,000	13,650,000
SUPPLEMENTAL NON-CASH DISCLOSURE:
Change in property and equipment not yet paid	2,853,000	(3,973,000)	6,644,000
Income taxes	2,121,000	26,766,000	16,190,000
Interest	$ 252,000	$ 179,000	$ 97,000