Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
OPERATING ACTIVITIES
Net income	$ 19,056	$ 7,916	$ 50,237	$ 36,568
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization			76,086	74,379
Deferred income taxes			8,268	3,165
Stock-based compensation			374	340
Gain on disposal of property and equipment	(7,156)	(7,471)	(15,410)	(19,845)
Changes in certain working capital items:
Trade receivables			8,773	8,778
Prepaid expenses and other current assets			387	(5,914)
Accounts payable, accrued liabilities, and accrued expenses			(18,168)	(13,221)
Accrued income taxes			(1,900)	(7,035)
Net cash provided by operating activities			108,647	77,215
INVESTING ACTIVITIES
Proceeds from sale of property and equipment			84,910	78,046
Purchases of property and equipment, net of trades			(127,526)	(104,883)
Acquisition of business, net of cash acquired			0	(87,635)
Change in other assets			563	(661)
Net cash used in investing activities			(42,053)	(115,133)
FINANCING ACTIVITIES
Payments of cash dividends			(4,950)	(5,000)
Shares withheld for employee taxes related to stock-based compensation			(181)	(198)
Repayments of debt assumed			0	(23,303)
Repurchases of common stock			(25,086)	0
Net cash used in financing activities			(30,217)	(28,501)
Net increase (decrease) in cash, cash equivalents and restricted cash			36,377	(66,419)
CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Beginning of period			106,098	150,225	$ 150,225
Ending Period	142,475	83,806	142,475	83,806	106,098
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds			4,974	21,753
Noncash investing and financing activities:
Purchased property and equipment in accounts payable			16,977	7,000
Sold revenue equipment in other current assets			15,900	6,313	900
RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Cash and Cash Equivalents, at Carrying Value	120,000	51,291	120,000	51,291	75,378
Restricted Cash and Investments, Current	3,298	11,600	3,298	11,600	7,900
Restricted Cash and Cash Equivalents, Noncurrent	$ 19,177	$ 20,915	$ 19,177	$ 20,915	$ 22,800