Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities:
Net income	$ 91,156	$ 48,024	$ 55,016
Noncash items included in net income:
Depreciation and amortization	87,102	76,240	65,020
Provision for doubtful accounts	2,634	1,475	1,463
Deferred income taxes	(20,776)	12,780	8,420
Loss (gain) from property disposals, net	(183)	716	310
Stock-based compensation	5,083	4,599	3,878
Changes in operating assets and liabilities, net of effects of acquisition:
Accounts receivable	(38,161)	(12,336)	6,896
Accounts payable	7,089	(3,414)	(382)
Other working capital items, net	14,023	13,399	(84)
Claims, insurance and other	4,531	(860)	(821)
Other, net	5,348	5,803	6,117
Net cash provided by operating activities	157,846	146,426	145,833
Investing Activities:
Acquisition of business, net of cash received			(22,238)
Acquisition of property and equipment	(186,696)	(119,365)	(86,499)
Proceeds from disposal of property and equipment	5,172	1,682	818
Net cash used in investing activities	(181,524)	(117,683)	(107,919)
Financing Activities:
Repayment of revolving credit agreement	(217,914)	(199,820)	(280,461)
Borrowing of revolving credit agreement	260,914	185,286	249,994
Proceeds from stock option exercises	4,480	3,173	2,533
Shares withheld for taxes	(1,250)	(650)	(3,119)
Repayment of senior notes	(7,143)	(7,143)	(7,143)
Repayment of capital leases	(12,228)	(8,174)	(3,507)
Other financing activity			(454)
Net cash provided by (used in) financing activities	26,859	(27,328)	(42,157)
Net Increase (Decrease) in Cash and Cash Equivalents	3,181	1,415	(4,243)
Cash and cash equivalents, beginning of year	1,539	124	4,367
Cash and cash equivalents, end of year	4,720	1,539	124
Non Cash Investing Activities
Equipment financed with capital leases	$ 35,483	$ 34,683	$ 27,054