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Receivables - Summary of Receivables (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Receivables [Abstract]      
Billed $ 157,333 $ 126,887  
Unbilled [1] 54,971 44,640  
Receivables, Gross, Current, Total 212,304 171,527  
Less: allowance for doubtful accounts (3,439) (2,941) $ (2,192)
Accounts receivable, net $ 208,865 $ 168,586  
[1] Represents revenue recorded in excess of amounts billable pursuant to contract provisions and generally become billable at contractually specified dates or upon the attainment of milestones. Unbilled amounts are expected to be realized within one year.