Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 141,096	$ 225,037	$ 261,006
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	205,188	188,535	157,081
Stock-based compensation	17,976	14,649	12,858
Excess tax benefit from share-based compensation	(28)	(1,477)	(6,221)
Deferred income taxes	33,442	40,477	70,131
Net loss/(gain) on sale and impairment of assets	32,484	(2,291)	(5,100)
Loss on settlement of foreign currency forward purchase contracts	0	18,686	0
Insurance proceeds received	0	0	5,527
Changes in operating assets and liabilities which provided/(used) cash, net of effect of acquisitions:
Accounts receivable, net	(15,952)	28,905	(39,107)
Materials and supplies	750	(4,073)	2,600
Prepaid expenses and other	836	7,462	17,451
Accounts payable and accrued expenses	(20,468)	(39,881)	14,703
Other assets and liabilities, net	11,715	(882)	535
Net cash provided by operating activities	407,039	475,147	491,464
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(219,544)	(371,504)	(331,499)
Grant proceeds from outside parties	36,094	41,742	27,980
Cash paid for acquisitions, net of cash acquired	(969,476)	(740,237)	(221,451)
Net payment from settlement of foreign currency forward purchase contracts related to an acquisition	0	(18,686)	0
Insurance proceeds for the replacement of assets	15,201	10,394	8,029
Proceeds from disposition of property and equipment	2,691	4,018	7,096
Net cash used in investing activities	(1,135,034)	(1,074,273)	(509,845)
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on revolving line-of-credit, long-term debt and capital lease obligations	(1,104,222)	(675,430)	(538,035)
Proceeds from revolving line-of-credit and long-term borrowings	1,074,516	1,261,640	543,300
Proceeds from noncontrolling interest	476,828	0	0
Proceeds from Class A common stock issuance	286,500	0	0
Stock issuance costs	(743)	0	0
Debt amendment/issuance costs	(17,731)	(9,622)	(3,880)
Proceeds from employee stock purchases	9,317	6,829	11,819
Excess tax benefits from share-based compensation	28	1,477	6,221
Treasury stock acquisitions	(4,541)	(3,261)	(4,186)
Net cash provided by financing activities	719,952	581,633	15,239
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	4,421	(6,293)	(7)
DECREASE IN CASH AND CASH EQUIVALENTS	(3,622)	(23,786)	(3,149)
CASH AND CASH EQUIVALENTS, beginning of year	35,941	59,727	62,876
CASH AND CASH EQUIVALENTS, end of year	$ 32,319	$ 35,941	$ 59,727