Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
OPERATING ACTIVITIES
Net income	$ 17,318	$ 13,378
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	22,451	25,783
Deferred income taxes	2,515	5,184
Stock-based compensation	284	123
Gain on disposal of property and equipment	(3,841)	(2,869)
Changes in certain working capital items:
Trade receivables	(4,324)	2,722
Prepaid expenses and other current assets	(396)	(667)
Accounts payable, accrued liabilities, and accrued expenses	(158)	(6,106)
Accrued income taxes	1,920	(5,321)
Net cash provided by operating activities	35,769	32,227
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	13,753	26,126
Purchases of property and equipment, net of trades	(34,209)	(32,018)
Change in other assets	4	436
Net cash used in investing activities	(20,452)	(5,456)
FINANCING ACTIVITIES
Payments of cash dividends	(1,640)	(1,667)
Shares withheld for employee taxes related to stock-based compensation	(107)	(97)
Repurchases of common stock	0	(1,293)
Net cash used in financing activities	(1,747)	(3,057)
Net increase in cash, cash equivalents and restricted cash	13,570	23,714
CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Beginning of period	182,938	106,098
Ending Period	196,508	129,812
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	235	46
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	7,309	27,355
Sold revenue equipment in other current assets	4,611	11,746
RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Cash and Cash Equivalents, at Carrying Value	176,303	105,019
Restricted Cash and Investments, Current	2,638	3,009
Restricted Cash and Cash Equivalents, Noncurrent	17,567	21,784
Ending Period	$ 196,508	$ 129,812