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Receivables - Summary of Receivables (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Receivables [Abstract]      
Billed $ 126,887 $ 124,731  
Unbilled [1] 44,640 45,247  
Receivables, Gross, Current, Total 171,527 169,978  
Less: allowance for doubtful accounts (2,941) (2,192) $ (2,126)
Receivables, net $ 168,586 $ 167,786  
[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.