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Accounts receivable (Tables)
12 Months Ended
Dec. 31, 2017
Accounts receivable [Abstract]  
Accounts receivable
Accounts receivable consist of the following:

 
December 31,
 
(in thousands)
2017
 
2016
 
Billed
 
$
118,918
  
$
90,439
 
Unbilled
  
24,735
   
48,039
 
Total
 
$
143,653
  
$
138,478