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Accounts Receivable - Accounts receivable, net (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Accounts Receivable    
Billed $ 135,201 $ 191,579
Unbilled 29,754 52,052
Other receivables 1,631 4,124
Total 166,586 247,755
Less: allowance for credit losses (4,815) (5,181)
Accounts receivable, net $ 161,771 $ 242,574