Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 75,417	$ 75,418
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	122,287	100,254
Stock-based compensation	8,857	9,525
Deferred income taxes	34,320	16,336
Net (gain)/loss on sale and impairment of assets	(781)	12,517
Changes in assets and liabilities which provided/(used) cash, net of effect of acquisitions:
Accounts receivable, net	10,066	(11,472)
Materials and supplies	2,198	(1,071)
Prepaid expenses and other	14,617	(2,078)
Accounts payable and accrued expenses	(48,282)	(46,236)
Other assets and liabilities, net	5,627	8,808
Net cash provided by operating activities	224,326	162,001
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(91,498)	(113,321)
Grant proceeds from outside parties	11,630	25,990
Cash paid for acquisitions, net of cash acquired	(102,655)	0
Proceeds from the sale of investment	2,100	0
Insurance proceeds for the replacement of assets	1,406	7,741
Proceeds from disposition of property and equipment	3,280	1,458
Net cash used in investing activities	(175,737)	(78,132)
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on revolving line-of-credit, long-term debt and capital lease obligations	(322,446)	(311,930)
Proceeds from revolving line-of-credit and long-term borrowings	320,191	215,434
Proceeds from employee stock purchases	4,962	3,135
Treasury stock purchases	(3,254)	(2,593)
Net cash used in financing activities	(547)	(95,954)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	3,382	1,115
INCREASE/(DECREASE) IN CASH AND CASH EQUIVALENTS	51,424	(10,970)
CASH AND CASH EQUIVALENTS, beginning of period	32,319	35,941
CASH AND CASH EQUIVALENTS, end of period	$ 83,743	$ 24,971