Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
OPERATING ACTIVITIES
Net income	$ 56,386	$ 73,056	$ 84,834
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	105,580	111,848	109,629
Deferred income taxes	(4,584)	8,618	39,067
Stock-based compensation, net of tax	856	1,251	1,134
Gain on disposal of property and equipment	(9,205)	(35,040)	(33,544)
Changes in certain working capital items:
Trade receivables	14,165	16,025	7,366
Prepaid expenses and other current assets	5,017	4,301	(1,009)
Accounts payable, accrued liabilities, and accrued expenses	(11,063)	202	(19,017)
Accrued income taxes	(1,371)	10,211	(16,007)
Net cash provided by operating activities	155,781	190,472	172,453
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	57,280	148,792	91,266
Purchases of property and equipment, net of trades	(86,088)	(217,253)	(204,973)
Payments for (Proceeds from) Delayed Tax Exempt Exchange	(9,335)	0	0
Acquisition of business, net of cash acquired	0	0	(3,011)
Change in other assets	(1,019)	1,248	1,239
Net cash (used in) provided by investing activities	(39,162)	(67,213)	(115,479)
FINANCING ACTIVITIES
Cash dividends paid	(6,666)	(6,942)	(7,034)
Proceeds from Lines of Credit	0	0	19,100
Repayments of Lines of Credit	0	(24,600)	(69,500)
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	0	(1,764)	0
Repurchases of common stock	(14,678)	(74,024)	0
Net cash used in financing activities	(21,344)	(107,330)	(57,434)
Net (decrease) increase in cash and cash equivalents	95,275	15,929	(460)
CASH AND CASH EQUIVALENTS
Beginning of period	33,232	17,303	17,763
End of period	128,507	33,232	17,303
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Interest paid	0	40	484
Cash paid during the period for income taxes, net of refunds	35,537	24,701	23,723
Noncash investing and financing activities:
Fair value of revenue equipment traded	0	0	3,393
Purchased property and equipment in accounts payable	63	1,217	230
Sold revenue equipment in other current assets	$ 160	$ 0	$ 0