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RECEIVABLES, CONTRACT ASSETS, AND DEFERRED REVENUE - Summary of Receivable (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Receivables [Abstract]    
Accounts receivable $ 159,324 $ 215,827
Unbilled receivables 228,603 207,155
Long-term unbilled receivables 108,048 113,278
Total receivables $ 495,975 $ 536,260