Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 128,802	$ 132,203
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	186,509	151,095
Stock-based compensation	13,354	13,835
Deferred income taxes	51,231	25,088
Net (gain)/loss on sale and impairment of assets	(1,096)	11,993
Changes in assets and liabilities which provided/(used) cash, net of effect of acquisitions:
Accounts receivable, net	(18,020)	(10,731)
Materials and supplies	8,998	(2,642)
Prepaid expenses and other	14,257	(1,930)
Accounts payable and accrued expenses	(41,529)	(29,484)
Other assets and liabilities, net	7,883	14,156
Net cash provided by operating activities	350,389	303,583
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(149,105)	(159,523)
Grant proceeds from outside parties	15,998	29,952
Cash paid for acquisitions, net of cash acquired	(107,586)	0
Proceeds from the sale of investment	2,100	0
Insurance proceeds for the replacement of assets	1,406	10,319
Proceeds from disposition of property and equipment	4,238	2,003
Net cash used in investing activities	(232,949)	(117,249)
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on revolving line-of-credit, long-term debt and capital lease obligations	(498,925)	(501,087)
Proceeds from revolving line-of-credit and long-term borrowings	418,735	300,495
Proceeds from employee stock purchases	8,003	5,969
Treasury stock purchases	(3,275)	(3,065)
Net cash used in financing activities	(75,462)	(197,688)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	5,980	1,779
INCREASE/(DECREASE) IN CASH AND CASH EQUIVALENTS	47,958	(9,575)
CASH AND CASH EQUIVALENTS, beginning of period	32,319	35,941
CASH AND CASH EQUIVALENTS, end of period	$ 80,277	$ 26,366