Consolidated Statements of Cash Flows	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
OPERATING ACTIVITIES
Net income	$ 72,677,000	$ 75,173,000	$ 56,386,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	101,329,000	103,905,000	105,580,000
Deferred income taxes	2,755,000	(27,121,000)	(4,584,000)
Stock-based Compensation	539,000	511,000	0
Amortization of stock-based compensation, net of tax	0	0	856,000
Gain on disposal of property and equipment	(24,963,000)	(26,674,000)	(9,205,000)
Changes in certain working capital items:
Trade receivables	15,338,000	15,239,000	14,165,000
Prepaid expenses and other current assets	1,227,000	860,000	5,017,000
Accounts payable, accrued liabilities, and accrued expenses	(26,012,000)	(26,893,000)	(11,063,000)
Accrued income taxes	3,653,000	(5,462,000)	(1,371,000)
Net cash provided by operating activities	146,543,000	109,538,000	155,781,000
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	130,752,000	147,578,000	57,280,000
Purchases of property and equipment, net of trades	(169,276,000)	(184,114,000)	(86,088,000)
Acquisition of business, net of cash acquired	0	(86,728,000)	0
Change in other assets	710,000	(233,000)	0
Net cash (used in) provided by investing activities	(37,814,000)	(123,497,000)	(28,808,000)
FINANCING ACTIVITIES
Cash dividends paid	(6,589,000)	(6,667,000)	(6,666,000)
Shares withheld for employee taxes related to stock-based compensation	(213,000)	(198,000)	0
Repayments on acquired debt	0	(23,303,000)	0
Repurchases of common stock	(25,087,000)	0	(14,678,000)
Net cash used in financing activities	(31,889,000)	(30,168,000)	(21,344,000)
Net increase (decrease) in cash, cash equivalents and restricted cash	76,840,000	(44,127,000)	105,629,000
CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Beginning of period	106,098,000	150,225,000	44,596,000
End of Period	182,938,000	106,098,000	150,225,000
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Interest paid	0	153,000	0
Cash paid during the period for income taxes, net of refunds	12,832,000	21,909,000	35,537,000
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	1,944,000	3,387,000	63,000
Sold revenue equipment in other current assets	3,783,000	869,000	160,000
RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH
Cash and Cash Equivalents	161,448,000	75,378,000	128,507,000
Restricted Cash included in other current assets	3,105,000	7,936,000	9,335,000
Restricted Cash included in other assets	18,385,000	22,784,000	12,383,000
End of Period	$ 182,938,000	$ 106,098,000	$ 150,225,000