Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 556,778	$ 141,096	$ 225,037
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	250,457	205,188	188,535
Stock-based compensation	17,554	17,976	14,649
Deferred income taxes	(319,249)	33,442	40,477
Net loss/(gain) on sale and impairment of assets	4,254	32,484	(2,291)
Loss on settlement of foreign currency forward purchase contracts	0	0	18,686
Changes in operating assets and liabilities which provided/(used) cash, net of effect of acquisitions:
Accounts receivable, net	(12,969)	(15,952)	28,905
Materials and supplies	2,474	750	(4,073)
Prepaid expenses and other	23,973	836	7,462
Accounts payable and accrued expenses	(35,341)	(20,468)	(39,881)
Other assets and liabilities, net	(8,725)	11,715	(882)
Net cash provided by operating activities	479,206	407,067	476,624
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(228,472)	(219,544)	(371,504)
Grant proceeds from outside parties	20,249	36,094	41,742
Cash paid for acquisitions, net of cash acquired	(107,586)	(969,476)	(740,237)
Net payment from settlement of foreign currency forward purchase contracts related to an acquisition	0	0	(18,686)
Proceeds from sale of investment	2,100	0	0
Insurance proceeds for the replacement of assets	1,590	15,201	10,394
Proceeds from disposition of property and equipment	5,225	2,691	4,018
Net cash used in investing activities	(306,894)	(1,135,034)	(1,074,273)
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on revolving line-of-credit, long-term debt and capital lease obligations	(661,561)	(1,104,222)	(675,430)
Proceeds from revolving line-of-credit and long-term borrowings	523,672	1,074,516	1,261,640
Proceeds from noncontrolling interest	0	476,828	0
Proceeds from Class A Common Stock issuance	0	286,500	0
Stock issuance costs	0	(743)	0
Debt amendment/issuance costs	0	(17,731)	(9,622)
Proceeds from employee stock purchases	11,583	9,317	6,829
Treasury stock acquisitions	(4,603)	(4,541)	(3,261)
Net cash (used in)/provided by financing activities	(130,909)	719,924	580,156
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	6,750	4,421	(6,293)
INCREASE/(DECREASE) IN CASH AND CASH EQUIVALENTS	48,153	(3,622)	(23,786)
CASH AND CASH EQUIVALENTS, beginning of year	32,319	35,941	59,727
CASH AND CASH EQUIVALENTS, end of year	$ 80,472	$ 32,319	$ 35,941