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SUPPLEMENTAL BALANCE SHEET INFORMATION - Activity in the Reserve for Bad Debts (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Reserve for bad debt:      
Balance at October 1, $ 6,217 $ 5,721 $ 2,696
Provision for bad debt 2,321 2,193 2,016
(Write-off) recovery of bad debt 1,389 (1,697) 1,009
Balance at September 30, $ 9,927 $ 6,217 $ 5,721