Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities:
Net income	$ 48,024	$ 55,016	$ 51,991
Noncash items included in net income:
Depreciation and amortization	76,240	65,020	59,022
Provision for doubtful accounts	1,475	1,463	1,942
Deferred income taxes	12,780	8,420	9,462
Loss (gain) from property disposals, net	716	310	(17)
Stock-based compensation	4,599	3,878	4,166
Changes in operating assets and liabilities:
Accounts receivable	(12,336)	6,896	(12,372)
Accounts payable	(3,414)	(382)	(8,590)
Other working capital items, net	13,399	(84)	(11,215)
Claims, insurance and other	(860)	(821)	5,260
Other, net	5,153	2,998	2,521
Net cash provided by operating activities	145,776	142,714	102,170
Investing Activities:
Acquisition of business, net of cash received		(22,238)
Acquisition of property and equipment	(119,365)	(86,499)	(97,750)
Proceeds from disposal of property and equipment	1,682	818	2,905
Net cash used in investing activities	(117,683)	(107,919)	(94,845)
Financing Activities:
Repayment of revolving credit agreement	(199,820)	(280,461)	(340,540)
Borrowing of revolving credit agreement	185,286	249,994	337,223
Proceeds from stock option exercises (including excess tax benefits)	3,173	2,533	7,623
Repayment of senior notes	(7,143)	(7,143)	(7,143)
Repayment of capital leases	(8,174)	(3,507)	(280)
Other financing activity		(454)
Net cash used in financing activities	(26,678)	(39,038)	(3,117)
Net Increase (Decrease) in Cash and Cash Equivalents	1,415	(4,243)	4,208
Cash and cash equivalents, beginning of period	124	4,367	159
Cash and cash equivalents, end of period	1,539	124	4,367
Non Cash Investing Activities
Equipment financed with capital leases	$ 34,683	$ 27,054	$ 16,886