Consolidate Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS PROVIDED BY OPERATING ACTIVITIES:
Net income	$ 33,464,000	$ 35,745,000	$ 29,834,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	82,445,000	75,323,000	68,243,000
Deferred income taxes	13,490,000	14,720,000	9,422,000
Tax benefits from share-based payment arrangement exercises	46,000	373,000	159,000
Excess tax benefits from share-based payment arrangement exercises	(235,000)	(432,000)	(115,000)
Allocated Share-based Compensation Expense	883,000	1,404,000	921,000
Equity in loss (earnings) from affiliate	1,018,000	296,000	(246,000)
Changes in other current operating items:
Receivables	8,518,000	7,968,000	(14,913,000)
Prepaid expenses and other	(1,173,000)	(1,274,000)	(1,586,000)
Accounts payable	594,000	(4,853,000)	(723,000)
Insurance and claims accruals	3,205,000	2,237,000	(406,000)
Accrued liabilities	1,813,000	6,419,000	(4,232,000)
Net cash provided by operating activities	133,566,000	128,238,000	81,971,000
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Other	(42,000)	(39,000)	(37,000)
Net cash used for investing activities	(97,290,000)	(125,081,000)	(118,606,000)
CASH FLOWS (USED FOR) PROVIDED BY FINANCING ACTIVITIES:
Borrowings under credit facility and long-term debt	179,687,000	141,908,000	168,444,000
Repayment of borrowings under credit facility and long-term debt	(209,668,000)	(128,414,000)	(144,071,000)
Repurchase and retirement of common stock	(7,513,000)	(16,175,000)
Dividends on common stock	(3,252,000)	(3,342,000)	(3,339,000)
Issuance of common stock from share-based payment arrangement exercises	4,416,000	3,490,000	1,214,000
Change in checks issued in excess of cash balances		(745,000)	745,000
Excess tax benefits from share-based payment arrangement exercises	235,000	432,000	115,000
Employee taxes paid in exchange for shares withheld	(127,000)
Net cash (used for) provided by financing activities	(36,222,000)	(2,846,000)	23,108,000
NET CHANGE IN CASH AND CASH EQUIVALENTS	54,000	311,000	(13,527,000)
CASH AND CASH EQUIVALENTS:
Beginning of year	434,000	123,000	13,650,000
End of year	488,000	434,000	123,000
SUPPLEMENTAL NON-CASH DISCLOSURE:
Change in property and equipment not yet paid	4,511,000	2,853,000	(3,973,000)
Income taxes	31,000	2,121,000	26,766,000
Interest	214,000	252,000	179,000
Revenue Equipment [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of assets	(10,502,000)	(5,584,000)	(4,387,000)
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(154,984,000)	(174,165,000)	(131,645,000)
Proceeds from revenue equipment dispositions	65,082,000	56,011,000	43,933,000
Facility [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on disposition of assets		(4,104,000)
Buildings And Land Office Equipment And Other [Member]
CASH FLOWS USED FOR INVESTING ACTIVITIES:
Revenue equipment additions	(7,369,000)	(13,482,000)	(30,885,000)
Proceeds from revenue equipment dispositions	$ 23,000	$ 6,594,000	$ 28,000