Consolidated Statements of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operations:
Net income	$ 29,834,000	$ 30,147,000	$ 27,267,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	68,243,000	64,476,000	60,904,000
Gain on disposition of revenue equipment	(4,387,000)	(5,972,000)	(5,302,000)
Deferred income taxes	9,422,000	4,303,000	4,822,000
Tax benefits from share-based payment arrangement exercises	159,000	189,000	409,000
Excess tax benefits from share-based payment arrangement exercises	(115,000)	(165,000)	(336,000)
Share-based payment arrangement compensation expense	921,000	1,084,000	1,225,000
Equity in earnings from affiliate	(246,000)	(111,000)
Income before income taxes attributable to noncontrolling interest		84,000	497,000
Changes in other current operating items:
Receivables	(14,913,000)	(6,478,000)	(7,795,000)
Prepaid expenses and other	(1,586,000)	(25,000)	(64,000)
Accounts payable	(723,000)	2,387,000	1,021,000
Insurance and claims accruals	(406,000)	566,000	796,000
Accrued liabilities	(4,232,000)	(1,298,000)	2,095,000
Net cash provided by operating activities	81,971,000	89,187,000	85,539,000
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Decrease in cash and cash equivalents resulting from change to equity method of accounting		(1,924,000)
Other	(37,000)	(35,000)	(34,000)
Net cash used for investing activities	(118,606,000)	(74,722,000)	(88,115,000)
CASH FLOWS PROVIDED BY (USED FOR) FINANCING ACTIVITIES:
Borrowings under credit facility and long-term debt	168,444,000	82,697,000	21,517,000
Repayment of borrowings under credit facility and long-term debt	(144,071,000)	(85,423,000)	(18,791,000)
Dividends on common stock	(3,339,000)	(2,769,000)	(18,679,000)
Issuance of common stock from share-based payment arrangement exercises	1,214,000	1,126,000	968,000
Change in net checks issued in excess of cash balances	745,000
Excess tax benefits from share-based payment arrangement exercises	115,000	165,000	336,000
Noncontrolling interest distributions		(84,000)	(123,000)
Net cash provided by (used for) financing activities	23,108,000	(4,288,000)	(14,772,000)
NET CHANGE IN CASH AND CASH EQUIVALENTS	(13,527,000)	10,177,000	(17,348,000)
CASH AND CASH EQUIVALENTS:
Beginning of year	13,650,000	3,473,000	20,821,000
End of year	123,000	13,650,000	3,473,000
SUPPLEMENTAL NON-CASH DISCLOSURE:
Change in property and equipment not yet paid for	(3,973,000)	6,644,000	(6,203,000)
Cash paid for:
Interest	179,000	97,000	9,000
Income taxes	26,766,000	16,190,000	8,836,000
Revenue Equipment [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Proceeds from disposition of property, plant, and equipment	43,933,000	48,048,000	46,634,000
Buildings And Land, Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Proceeds from disposition of property, plant, and equipment	28,000	28,000	638,000
Revenue Equipment [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Additions to property, plant, and equipment	(131,645,000)	(102,808,000)	(121,777,000)
Buildings And Land, Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Additions to property, plant, and equipment	$ (30,885,000)	$ (18,031,000)	$ (13,576,000)