Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating Activities:
Net income	$ 51,406	$ 28,966
Noncash items included in net income:
Depreciation and amortization	48,271	42,269
Other, net	9,395	14,380
Changes in operating assets and liabilities, net	3,046	(6,298)
Net cash provided by operating activities	112,118	79,317
Investing Activities:
Acquisition of property and equipment	(118,573)	(127,330)
Proceeds from disposal of property and equipment	418	923
Net cash used in investing activities	(118,155)	(126,407)
Financing Activities:
Repayment of revolving credit agreement	(92,475)	(70,386)
Borrowing of revolving credit agreement	99,475	125,400
Proceeds from stock option exercises	4,165	1,288
Shares withheld for taxes	(1,321)	(1,211)
Repayment of senior notes		(3,571)
Repayment of capital leases	(7,338)	(5,570)
Net cash provided by financing activities	2,506	45,950
Net Decrease in Cash and Cash Equivalents	(3,531)	(1,140)
Cash and cash equivalents, beginning of period	4,720	1,539
Cash and cash equivalents, end of period	1,189	399
Non Cash Investing Activities
Equipment financed with capital leases	$ 22,422	$ 28,691