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Revenue Recognition - Accounts Receivable (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Sep. 30, 2020
Accounts receivable      
Billed $ 98,116   $ 161,473
Allowance for doubtful accounts (1,359)   (1,498)
Accounts receivable - net 96,757   159,975
Trade receivables sold to a financial institution 6,400   18,400
Receipt of withheld proceeds from sale of trade receivables 1,842 $ 5,521  
U.S. federal government contracts      
Accounts receivable      
Amounts billed $ 26,400   $ 82,200