Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
OPERATING ACTIVITIES
Net income	$ 13,378	$ 14,036
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	25,783	22,940
Deferred income taxes	5,184	53
Stock-based compensation	123	113
Gain on disposal of property and equipment	(2,869)	(6,075)
Changes in certain working capital items:
Trade receivables	2,722	623
Prepaid expenses and other current assets	(667)	(3,351)
Accounts payable, accrued liabilities, and accrued expenses	(6,106)	(1,814)
Accrued income taxes	(5,321)	5,574
Net cash provided by operating activities	32,227	32,099
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	26,126	21,542
Purchases of property and equipment, net of trades	(32,018)	(17,617)
Change in other assets	436	0
Net cash (used in) provided by investing activities	(5,456)	3,925
FINANCING ACTIVITIES
Payments of cash dividends	(1,667)	(1,667)
Shares withheld for employee taxes related to stock-based compensation	(97)	(63)
Repurchases of common stock	(1,293)	0
Net cash used in financing activities	(3,057)	(1,730)
Net increase in cash, cash equivalents and restricted cash	23,714	34,294
CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Beginning of period	106,098	150,225
End of period	129,812	184,519
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid (received) during the period for income taxes, net of refunds	46	(12)
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	27,355	3,406
Sold revenue equipment in other current assets	11,746	2,602
RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Cash and Cash Equivalents, at Carrying Value	105,019	159,170
Restricted Cash and Investments, Current	3,009	13,159
Restricted Cash and Cash Equivalents, Noncurrent	$ 21,784	$ 12,190