Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities:
Net income	$ 104,981	$ 91,129	$ 48,024
Noncash items included in net income:
Depreciation and amortization	102,153	87,102	76,240
Provision for doubtful accounts	1,978	2,634	1,475
Deferred income taxes	27,470	(20,776)	12,780
Loss (gain) from property disposals, net	379	(183)	716
Stock-based compensation	5,619	5,083	4,599
Changes in operating assets and liabilities, net of effects of acquisition:
Accounts receivable	(12,981)	(37,985)	(12,401)
Accounts payable	10,608	6,940	(3,349)
Other working capital items, net	15,537	14,023	13,399
Claims, insurance and other	(2,740)	4,531	(860)
Other, net	3,432	5,348	5,803
Net cash provided by operating activities	256,436	157,846	146,426
Investing Activities:
Acquisition of property and equipment	(223,672)	(186,696)	(119,365)
Proceeds from disposal of property and equipment	1,088	5,172	1,682
Net cash used in investing activities	(222,584)	(181,524)	(117,683)
Financing Activities:
Repayment of revolving credit agreement	(233,888)	(217,914)	(199,820)
Borrowing of revolving credit agreement	210,888	260,914	185,286
Proceeds from stock option exercises	4,165	4,480	3,173
Shares withheld for taxes	(1,396)	(1,250)	(650)
Repayment of senior notes		(7,143)	(7,143)
Repayment of capital leases	(16,147)	(12,228)	(8,174)
Net cash provided by (used in) financing activities	(36,378)	26,859	(27,328)
Net Increase (Decrease) in Cash and Cash Equivalents	(2,526)	3,181	1,415
Cash and cash equivalents, beginning of year	4,720	1,539	124
Cash and cash equivalents, end of year	2,194	4,720	1,539
Non Cash Investing Activities
Equipment financed with capital leases	$ 29,090	$ 35,483	$ 34,683