Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 15,791	$ 488
Receivables:
Trade, less allowances of $300 and $275, respectively	74,886	69,199
Other	6,131	4,436
Prepaid expenses and other	19,810	19,307
Total current assets	116,618	93,430
Property and equipment:
Less accumulated depreciation	(211,728)	(201,728)
Net property and equipment	571,920	557,825
Other assets	1,865	2,493
	690,403	653,748
Current liabilities:
Accounts payable	16,478	14,391
Insurance and claims accruals	26,177	19,440
Accrued liabilities	21,622	26,839
Total current liabilities	64,277	60,670
Long-term debt		7,886
Deferred income taxes	100,626	147,854
Total liabilities	164,903	216,410
Commitments and contingencies (Note 16)
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; 54,533,455 shares at December 31, 2017, and 54,391,525 shares at December 31, 2016, issued and outstanding	545	544
Additional paid-in capital	76,413	74,175
Retained earnings	448,542	362,619
Total stockholders’ equity	525,500	437,338
	690,403	653,748
Revenue Equipment [Member]
Property and equipment:
Property and equipment	652,974	634,831
Land and Building [Member]
Property and equipment:
Property and equipment	79,881	75,566
Buildings And Land Office Equipment And Other [Member]
Property and equipment:
Property and equipment	$ 50,793	$ 49,156