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Accounts Receivable - Summary of components of accounts receivable (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Trade receivables:    
Billed $ 266,660 $ 296,588
Unbilled 56,408 68,494
Other receivables 5,278 17,242
Total 328,346 382,324
Less: allowance for doubtful accounts (4,813) (4,471)
Accounts receivable, net $ 323,533 $ 377,853