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Accounts Receivable and Allowance for Expected Credit Losses - Accounts Receivable, Net (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Accounts receivable:    
Billed receivables $ 542,247 $ 513,459
Unbilled receivables 294,194 236,285
Allowance for expected credit losses (37,925) (38,387)
Accounts receivable, net $ 798,516 $ 711,357