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RECEIVABLES (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Jan. 01, 2018
[1]
Dec. 31, 2017
Receivables [Abstract]      
Trade accounts receivable $ 246,941   $ 244,133
Unbilled receivables 91,580   69,138
Allowance for doubtful accounts (15,352)   (11,308)
Total $ 323,169 [1] $ 301,963 $ 301,963
[1] Includes billed and unbilled amounts, net of allowance for doubtful accounts. See Note 3 for details.