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Revenue Recognition - Accounts Receivable (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Accounts receivable    
Billed $ 161,473 $ 127,406
Allowance for doubtful accounts (1,498) (1,392)
Accounts receivable - net 159,975 126,014
Trade receivables sold to a financial institution 18,400 31,100
Receipt of withheld proceeds from sale of trade receivables 5,521 5,500
U.S. federal government contracts    
Accounts receivable    
Amounts billed $ 82,200 $ 60,300