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REVENUES - Contract balances (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Accounts receivables, net        
Disaggregation of revenue:        
Unbilled trade receivables $ 95,898 $ 56,408 $ 113,453 $ 68,494