Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES
Net income	$ 75,173	$ 56,386	$ 73,056
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	103,905	105,580	111,848
Deferred income taxes	(27,121)	(4,584)	8,618
Stock-based Compensation	511	0	0
Amortization of stock-based compensation, net of tax	0	856	1,251
Gain on disposal of property and equipment	(26,674)	(9,205)	(35,040)
Changes in certain working capital items:
Trade receivables	15,239	14,165	16,025
Prepaid expenses and other current assets	860	5,017	4,301
Accounts payable, accrued liabilities, and accrued expenses	(26,893)	(11,063)	202
Accrued income taxes	(5,462)	(1,371)	10,211
Net cash provided by operating activities	109,538	155,781	190,472
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	147,578	57,280	148,792
Purchases of property and equipment, net of trades	(184,114)	(86,088)	(217,253)
Payments for (Proceeds from) Delayed Tax Exempt Exchange	4,843	(9,335)	0
Acquisition of business, net of cash acquired	(86,728)	0	0
Change in designated funds for claims liabilities	(13,899)	0	0
Change in other assets	(179)	(1,019)	1,248
Net cash (used in) provided by investing activities	(132,499)	(39,162)	(67,213)
FINANCING ACTIVITIES
Cash dividends paid	(6,667)	(6,666)	(6,942)
Shares withheld for employee taxes related to stock-based compensation	(198)	0	0
Repayments of Lines of Credit	(23,303)	0	(24,600)
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability	0	0	(1,764)
Repurchases of common stock	0	(14,678)	(74,024)
Net cash used in financing activities	(30,168)	(21,344)	(107,330)
Net (decrease) increase in cash and cash equivalents	(53,129)	95,275	15,929
CASH AND CASH EQUIVALENTS
Beginning of period	128,507	33,232	17,303
End of period	75,378	128,507	33,232
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Interest paid	153	0	40
Cash paid during the period for income taxes, net of refunds	21,909	35,537	24,701
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	3,387	63	1,217
Sold revenue equipment in other current assets	$ 869	$ 160	$ 0