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Receivables - Activity of Allowance for Doubtful Accounts (Detail) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance, beginning of year $ 2,192 $ 2,126  
Add: expense 1,640 2,011 $ 0
Less: write-offs (net of recoveries) (891) (1,945)  
Balance, end of year $ 2,941 $ 2,192 $ 2,126