Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
OPERATING ACTIVITIES
Net income	$ 17,803	$ 14,616	$ 31,182	$ 28,653
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization			50,743	45,565
Deferred income taxes			4,367	1,189
Stock-based compensation			319	281
Gain on disposal of property and equipment	(5,386)	(6,299)	(8,254)	(12,375)
Changes in certain working capital items:
Trade receivables			5,810	3,000
Prepaid expenses and other current assets			(2,087)	(5,356)
Accounts payable, accrued liabilities, and accrued expenses			(11,073)	(7,650)
Accrued income taxes			(1,286)	(2,474)
Net cash provided by operating activities			69,721	50,833
INVESTING ACTIVITIES
Proceeds from sale of property and equipment			64,359	48,955
Purchases of property and equipment, net of trades			(86,936)	(50,213)
Change in other assets			460	(678)
Net cash used in investing activities			(22,117)	(1,936)
FINANCING ACTIVITIES
Payments of cash dividends			(3,311)	(3,334)
Shares withheld for employee taxes related to stock-based compensation			(124)	(118)
Repurchases of common stock			(20,922)	0
Net cash used in financing activities			(24,357)	(3,452)
Net increase in cash, cash equivalents and restricted cash			23,247	45,445
CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Beginning of period			106,098	150,225	$ 150,225
Ending Period	129,345	195,670	129,345	195,670	106,098
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds			1,598	14,023
Noncash investing and financing activities:
Purchased property and equipment in accounts payable			511	1,163
Sold revenue equipment in other current assets			5,600	3,904	900
RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Cash and Cash Equivalents, at Carrying Value	106,402	171,292	106,402	171,292	75,378
Restricted Cash and Investments, Current	2,819	12,857	2,819	12,857	7,900
Restricted Cash and Cash Equivalents, Noncurrent	$ 20,124	$ 11,521	$ 20,124	$ 11,521	$ 22,800