Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 56,763	$ 15,791
Receivables:
Trade, less allowances of $348 and $300, respectively	83,033	74,886
Other	3,808	6,131
Prepaid expenses and other	19,924	19,810
Total current assets	163,528	116,618
Property and equipment:
Less accumulated depreciation	(228,200)	(211,728)
Net property and equipment	588,230	571,920
Other assets	2,146	1,865
	753,904	690,403
Current liabilities:
Accounts payable	15,704	16,478
Insurance and claims accruals	28,103	26,177
Accrued liabilities	28,166	21,622
Total current liabilities	71,973	64,277
Deferred income taxes	105,977	100,626
Total liabilities	177,950	164,903
Commitments and contingencies (Note 18)
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; 192,000,000 shares authorized; 54,466,691 shares at December 31, 2018, and 54,533,455 shares at December 31, 2017, issued and outstanding	545	545
Additional paid-in capital	76,814	76,413
Retained earnings	498,595	448,542
Total stockholders’ equity	575,954	525,500
	753,904	690,403
Revenue Equipment [Member]
Property and equipment:
Property and equipment	679,667	652,974
Land and Building [Member]
Property and equipment:
Property and equipment	85,578	79,881
Buildings And Land Office Equipment And Other [Member]
Property and equipment:
Property and equipment	$ 51,185	$ 50,793