Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CASH FLOWS FROM OPERATING ACTIVITIES:
NET INCOME	$ 225,037,000	$ 260,755,000	$ 271,296,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	188,535,000	157,081,000	141,644,000
Stock-based compensation	14,649,000	12,858,000	16,951,000
Excess tax benefit from share-based compensation	(1,477,000)	(6,221,000)	(6,861,000)
Deferred income taxes	40,477,000	70,131,000	10,229,000
Net gain on sale of assets	(2,291,000)	(5,100,000)	(4,677,000)
Loss on settlement of foreign currency forward purchase contracts	18,686,000	0	0
Insurance proceeds received	0	5,527,000	11,053,000
Changes in operating assets and liabilities which provided/(used) cash, net of effect of acquisitions:
Accounts receivable, net	28,905,000	(39,107,000)	(47,780,000)
Materials and supplies	(4,073,000)	2,600,000	(1,839,000)
Prepaid expenses and other	7,462,000	17,451,000	3,304,000
Accounts payable and accrued expenses	(39,881,000)	14,703,000	16,383,000
Other assets and liabilities, net	(882,000)	786,000	3,801,000
Net cash provided by operating activities	475,147,000	491,464,000	413,504,000
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(371,504,000)	(331,499,000)	(249,318,000)
Grant proceeds from outside parties	41,742,000	27,980,000	33,913,000
Cash paid for acquisitions, net of cash acquired	(740,237,000)	(221,451,000)	0
Net payment from settlement of foreign currency forward purchase contracts related to an acquisition	(18,686,000)	0	0
Insurance proceeds for the replacement of assets	10,394,000	8,029,000	0
Proceeds from disposition of property and equipment	4,018,000	7,096,000	6,687,000
Net cash used in investing activities	(1,074,273,000)	(509,845,000)	(208,718,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on long-term borrowings, including capital leases	(675,430,000)	(538,035,000)	(471,957,000)
Proceeds from issuance of long-term debt	1,261,640,000	543,300,000	262,651,000
Debt amendment/issuance costs	(9,622,000)	(3,880,000)	(2,773,000)
Dividends paid on Series A-1 Preferred Stock	0	0	(2,139,000)
Proceeds from employee stock purchases	6,829,000	11,819,000	12,510,000
Treasury stock acquisitions	(3,261,000)	(4,186,000)	(11,095,000)
Excess tax benefit from share-based compensation	1,477,000	6,221,000	6,861,000
Net cash provided by/(used in) financing activities	581,633,000	15,239,000	(205,942,000)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(6,293,000)	(7,000)	(740,000)
DECREASE IN CASH AND CASH EQUIVALENTS	(23,786,000)	(3,149,000)	(1,896,000)
CASH AND CASH EQUIVALENTS, beginning of year	59,727,000	62,876,000	64,772,000
CASH AND CASH EQUIVALENTS, end of year	$ 35,941,000	$ 59,727,000	$ 62,876,000