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COMMITMENTS AND CONTINGENCIES - Contractual obligations and commercial commitments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2013
COMMITMENTS AND CONTINGENCIES        
Outstanding letters of credit $ 20,900      
Outstanding purchase commitments, including agreements that are non-cancelable and cancelable 106,300      
Uncertain Tax Positions        
Gross unrecognized tax benefits 18,728 $ 20,137 $ 22,694 $ 45,382
Unrecognized tax benefits that would impact the annual effective tax rate if recognized 2,061 $ 3,470 $ 6,027  
Unrecognized tax benefits if recognized, would impact certain deferred tax assets $ 16,700