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RECEIVABLES
6 Months Ended
Jun. 30, 2018
Receivables [Abstract]  
RECEIVABLES
RECEIVABLES
A summary of accounts receivable as of June 30, 2018 and December 31, 2017 is as follows (in thousands):
 
 
June 30, 2018
 
December 31, 2017
 
(unaudited)
 
 
Trade accounts receivable
$
246,941

 
$
244,133

Unbilled receivables
91,580

 
69,138

Allowance for doubtful accounts
(15,352
)
 
(11,308
)
Total
$
323,169

 
$
301,963