Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 27,289	$ 27,019
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	60,774	49,330
Stock-based compensation	4,213	5,074
Deferred income taxes	13,572	5,029
Net (gain)/loss on sale and impairment of assets	(427)	12,825
Changes in assets and liabilities which provided/(used) cash, net of effect of acquisitions:
Accounts receivable, net	6,524	9,617
Materials and supplies	(2,140)	(1,381)
Prepaid expenses and other	(2,226)	(6,564)
Accounts payable and accrued expenses	(29,330)	(34,956)
Other assets and liabilities, net	6,767	4,048
Net cash provided by operating activities	85,016	70,041
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(34,738)	(47,912)
Grant proceeds from outside parties	4,771	16,229
Proceeds from Previous Acquisition	2,935	0
Insurance proceeds for the replacement of assets	1,406	2,418
Proceeds from disposition of property and equipment	928	292
Net cash used in investing activities	(24,698)	(28,973)
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on revolving line-of-credit, long-term debt and capital lease obligations	(167,730)	(143,684)
Proceeds from revolving line-of-credit and long-term borrowings	112,294	104,316
Proceeds from employee stock purchases	2,348	2,668
Treasury stock purchases	(3,162)	(1,974)
Net cash used in financing activities	(56,250)	(38,674)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	1,878	778
INCREASE IN CASH AND CASH EQUIVALENTS	5,946	3,172
CASH AND CASH EQUIVALENTS, beginning of period	32,319	35,941
CASH AND CASH EQUIVALENTS, end of period	$ 38,265	$ 39,113