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FINANCING ARRANGEMENTS (Summary of Financing Arrangements) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Debt Instrument [Line Items]
Current portion of long-term obligations	$ 4,000		$ 4,000		$ 0
Long-term obligations	1,640,000		1,640,000		1,645,000
Unamortized debt issuance costs and debt discount/premium, net	(14,029)		(14,029)		(11,728)
Long-term obligations, at carrying value	1,625,971		1,625,971		1,633,272
Total current and long-term obligations, at carrying value	1,629,971		1,629,971		1,633,272
Service revenues	612,352	$ 594,225	1,783,506	$ 1,709,018
Secured debt \| Term Loan Agreement
Debt Instrument [Line Items]
Current portion of long-term obligations	4,000		4,000		0
Long-term obligations	395,000		395,000		0
Unsecured debt \| Senior unsecured notes, at 5.25%, due August 1, 2020 (2020 Notes)
Debt Instrument [Line Items]
Long-term obligations	$ 400,000		$ 400,000		800,000
Interest rate	5.25%		5.25%
Unsecured debt \| Senior unsecured notes, at 5.125%, due June 1, 2021 (2021 Notes)
Debt Instrument [Line Items]
Long-term obligations	$ 845,000		$ 845,000		$ 845,000
Interest rate	5.125%		5.125%