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Accounts Receivable - Summary of components of accounts receivable (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Trade receivables:    
Billed $ 296,588 $ 122,216
Unbilled 68,494 39,223
Other receivables 17,242 10,280
Total 382,324 171,719
Less: allowance for doubtful accounts (4,471) (2,553)
Accounts receivable, net $ 377,853 $ 169,166