Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 27,019	$ 23,904
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	49,330	41,813
Stock-based compensation	5,074	3,881
Excess tax benefit from share-based compensation	(53)	(834)
Deferred income taxes	5,029	10,236
Net loss/(gain) on sale and impairment of assets	12,825	(317)
Loss on settlement of foreign currency forward purchase contracts	0	18,686
Changes in assets and liabilities which provided/(used) cash, net of effect of acquisitions:
Accounts receivable, net	9,617	20,547
Materials and supplies	(1,381)	543
Prepaid expenses and other	(6,564)	(3,410)
Accounts payable and accrued expenses	(26,877)	(32,133)
Other assets and liabilities, net	4,048	1,078
Net cash provided by operating activities	78,067	83,994
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(55,993)	(66,267)
Grant proceeds from outside parties	16,229	16,369
Cash paid for acquisitions, net of cash acquired	0	(723,944)
Net payment from settlement of foreign currency forward purchase contracts related to an acquisition	0	(18,686)
Insurance proceeds for the replacement of assets	2,418	1,421
Proceeds from disposition of property and equipment	292	1,082
Net cash used in investing activities	(37,054)	(790,025)
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on revolving line-of-credit, long-term debt and capital lease obligations	(143,684)	(125,307)
Proceeds from revolving line-of-credit and long-term borrowings	104,316	892,805
Debt amendment/issuance costs	0	(5,933)
Proceeds from employee stock purchases	2,668	2,727
Treasury stock purchases	(1,974)	(2,847)
Excess tax benefit from share-based compensation	53	834
Net cash (used in)/provided by financing activities	(38,621)	762,279
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	780	(3,555)
INCREASE IN CASH AND CASH EQUIVALENTS	3,172	52,693
CASH AND CASH EQUIVALENTS, beginning of period	35,941	59,727
CASH AND CASH EQUIVALENTS, end of period	$ 39,113	$ 112,420