Consolidated Statements of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operations:
Net income	$ 27,267,000	$ 24,285,000	$ 19,742,000
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation	60,904,000	57,531,000	51,916,000
Gain on disposition of revenue equipment	(5,302,000)	(3,800,000)	(1,110,000)
Deferred income taxes	4,822,000	10,076,000	10,550,000
Tax benefits from share-based payment arrangement exercises	409,000	164,000	385,000
Excess tax benefits from share-based payment arrangement exercises	(336,000)	(142,000)	(338,000)
Share-based payment arrangement compensation expense	1,225,000	1,349,000	1,264,000
Income before income taxes attributable to noncontrolling interest	497,000	807,000	631,000
Changes in other current operating items:
Receivables	(7,795,000)	1,247,000	(14,246,000)
Prepaid expenses and other	(64,000)	(2,020,000)	(1,078,000)
Accounts payable	1,021,000	(1,585,000)	348,000
Insurance and claims accruals	796,000	(4,611,000)	(1,569,000)
Accrued liabilities	2,095,000	2,905,000	(1,973,000)
Net cash provided by operating activities	85,539,000	86,206,000	64,522,000
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Other	(34,000)	102,000	(26,000)
Net cash used for investing activities	(88,115,000)	(48,271,000)	(81,181,000)
CASH FLOWS (USED FOR) PROVIDED BY FINANCING ACTIVITIES:
Borrowings under credit facility and long-term debt	21,517,000	37,632,000	160,395,000
Repayment of borrowings under credit facility and long-term debt	(18,791,000)	(56,978,000)	(142,548,000)
Dividends on common stock	(18,679,000)	(1,758,000)	(877,000)
Issuance of common stock from share-based payment arrangement exercises	968,000	137,000	303,000
Excess tax benefits from share-based payment arrangement exercises	336,000	142,000	338,000
Noncontrolling interest distributions	(123,000)	(529,000)	(451,000)
Change in net checks issued in excess of cash balances		(1,066,000)	(605,000)
Net cash (used for) provided by financing activities	(14,772,000)	(22,420,000)	16,555,000
NET CHANGE IN CASH AND CASH EQUIVALENTS	(17,348,000)	15,515,000	(104,000)
CASH AND CASH EQUIVALENTS:
Beginning of year	20,821,000	5,306,000	5,410,000
End of year	3,473,000	20,821,000	5,306,000
Cash paid (received) for:
Interest	9,000	36,000	225,000
Income taxes	8,836,000	(2,559,000)	13,789,000
Non-cash investing activities:
Change in property and equipment not yet paid for	(6,203,000)	1,499,000	253,000
Revenue Equipment [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Additions to property, plant, and equipment	(121,777,000)	(75,332,000)	(109,345,000)
Buildings And Land, Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Additions to property, plant, and equipment	(13,576,000)	(9,573,000)	(11,959,000)
Revenue Equipment [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Proceeds from disposition of property, plant, and equipment	46,634,000	36,457,000	40,062,000
Buildings And Land, Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Proceeds from disposition of property, plant, and equipment	$ 638,000	$ 75,000	$ 87,000