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Contracts with Customers - Summary of Accounts Receivable, Net (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Revenue From Contract With Customer [Abstract]    
Billed $ 443,024 $ 512,357
Unbilled 188,428 190,222
Total accounts receivable, gross 631,452 702,579
Allowance for doubtful accounts (4,007) (4,001)
Total accounts receivable, net $ 627,445 $ 698,578