Consolidated Statements of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operations:
Net income	$ 30,147,000	$ 27,267,000	$ 24,285,000
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation	64,476,000	60,904,000	57,531,000
Gain on disposition of revenue equipment	(5,972,000)	(5,302,000)	(3,800,000)
Deferred income taxes	4,303,000	4,822,000	10,076,000
Tax benefits from share-based payment arrangement exercises	189,000	409,000	164,000
Excess tax benefits from share-based payment arrangement exercises	(165,000)	(336,000)	(142,000)
Share-based payment arrangement compensation expense	1,084,000	1,225,000	1,349,000
Income before income taxes attributable to noncontrolling interest	84,000	497,000	807,000
Equity in earnings from affiliate	(111,000)
Changes in other current operating items:
Receivables	(6,478,000)	(7,795,000)	1,247,000
Prepaid expenses and other	(25,000)	(64,000)	(2,020,000)
Accounts payable	2,387,000	1,021,000	(1,585,000)
Insurance and claims accruals	566,000	796,000	(4,611,000)
Accrued liabilities	(1,298,000)	2,095,000	2,905,000
Net cash provided by operating activities	89,187,000	85,539,000	86,206,000
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Decrease in cash and cash equivalents resulting from change to equity method of accounting	(1,924,000)
Other	(35,000)	(34,000)	102,000
Net cash used for investing activities	(74,722,000)	(88,115,000)	(48,271,000)
CASH FLOWS USED FOR FINANCING ACTIVITIES:
Borrowings under credit facility and long-term debt	82,697,000	21,517,000	37,632,000
Repayment of borrowings under credit facility and long-term debt	(85,423,000)	(18,791,000)	(56,978,000)
Dividends on common stock	(2,769,000)	(18,679,000)	(1,758,000)
Issuance of common stock from share-based payment arrangement exercises	1,126,000	968,000	137,000
Excess tax benefits from share-based payment arrangement exercises	165,000	336,000	142,000
Noncontrolling interest distributions	(84,000)	(123,000)	(529,000)
Change in net checks issued in excess of cash balances			(1,066,000)
Net cash used for financing activities	(4,288,000)	(14,772,000)	(22,420,000)
NET CHANGE IN CASH AND CASH EQUIVALENTS	10,177,000	(17,348,000)	15,515,000
CASH AND CASH EQUIVALENTS:
Beginning of year	3,473,000	20,821,000	5,306,000
End of year	13,650,000	3,473,000	20,821,000
SUPPLEMENTAL NON-CASH DISCLOSURE:
Change in property and equipment not yet paid for	6,644,000	(6,203,000)	1,499,000
Cash paid (received) for:
Interest	97,000	9,000	36,000
Income taxes	16,190,000	8,836,000	(2,559,000)
Revenue Equipment [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Additions to property, plant, and equipment	(102,808,000)	(121,777,000)	(75,332,000)
Buildings And Land, Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Additions to property, plant, and equipment	(18,031,000)	(13,576,000)	(9,573,000)
Revenue Equipment [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Proceeds from disposition of property, plant, and equipment	48,048,000	46,634,000	36,457,000
Buildings And Land, Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Proceeds from disposition of property, plant, and equipment	$ 28,000	$ 638,000	$ 75,000