Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
OPERATING ACTIVITIES
Net income	$ 73,056	$ 84,834	$ 70,582
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	111,848	109,629	69,649
Deferred income taxes	8,618	39,067	10,262
Loss on sale of investments	0	0	200
Stock-based compensation, net of tax	1,251	1,134	1,184
Gain on disposal of property and equipment	(35,040)	(33,544)	(33,270)
Changes in certain working capital items:
Trade receivables	16,025	7,366	7,834
Prepaid expenses and other current assets	4,301	(1,009)	904
Accounts payable, accrued liabilities, and accrued expenses	202	(19,017)	(9,722)
Accrued income taxes	10,211	(16,007)	(6,388)
Net cash provided by operating activities	190,472	172,453	111,235
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	148,792	91,266	92,313
Purchases of property and equipment, net of trades	(217,253)	(204,973)	(135,195)
Maturity, calls and sales of investments	0	0	21,100
Acquisition of business, net of cash acquired	0	(3,011)	(110,900)
Change in other assets	1,248	1,239	(825)
Net cash (used in) provided by investing activities	(67,213)	(115,479)	(133,507)
FINANCING ACTIVITIES
Cash dividends paid	(6,942)	(7,034)	(6,861)
Proceeds from Lines of Credit	0	19,100	75,000
Repayments of Lines of Credit	(24,600)	(69,500)	0
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	(1,764)	0	0
Repayments on debt assumed	0	0	(147,942)
Repurchases of common stock	(74,024)	0	0
Net cash used in financing activities	(107,330)	(57,434)	(79,803)
Net (decrease) increase in cash and cash equivalents	15,929	(460)	(102,075)
CASH AND CASH EQUIVALENTS
Beginning of period	17,303	17,763	119,838
End of period	33,232	17,303	17,763
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Interest paid	40	484	4
Cash paid during the period for income taxes, net of refunds	24,701	23,723	38,101
Noncash investing and financing activities:
Fair value of revenue equipment traded	0	3,393	2,138
Purchased property and equipment in accounts payable	1,217	230	11,191
Issuance of common stock in acquisition of business	$ 0	$ 0	$ 41,100