Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 488	$ 434
Receivables:
Trade, less allowances of $275 and $305, respectively	69,199	70,597
Other	4,436	10,885
Prepaid expenses and other	19,307	18,134
Total current assets	93,430	100,050
Property and equipment:
Less accumulated depreciation	(201,728)	(196,588)
Net property and equipment	557,825	528,009
Other assets	2,493	3,469
	653,748	631,528
Current liabilities:
Accounts payable	14,391	12,007
Insurance and claims accruals	19,440	16,235
Accrued liabilities	26,839	21,634
Total current liabilities	60,670	49,876
Long-term debt	7,886	37,867
Deferred income taxes	147,854	134,364
Total liabilities	216,410	222,107
Commitments and contingencies (Note 14)
Stockholders’ equity:
Preferred stock, $.01 par value per share; 2,000,000 shares authorized; no shares issued and outstanding
Common stock, $.01 par value per share; 96,000,000 shares authorized; 32,634,915 shares at December 31, 2016, and 32,759,806 shares at December 31, 2015, issued and outstanding	326	328
Additional paid-in capital	74,175	76,468
Retained earnings	362,837	332,625
Total stockholders’ equity	437,338	409,421
	653,748	631,528
Revenue Equipment [Member]
Property and equipment:
Property and equipment	634,831	606,144
Land and Building [Member]
Property and equipment:
Property and equipment	75,566	73,159
Buildings And Land Office Equipment And Other [Member]
Property and equipment:
Property and equipment	$ 49,156	$ 45,294