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SUPPLEMENTAL BALANCE SHEET INFORMATION - Activity in the Reserve for Bad Debts (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Reserve for bad debt:      
Balance at October 1, $ 9,927 $ 6,217 $ 5,721
Provision for bad debt 2,203 2,321 2,193
(Write-off) recovery of bad debt (10,310) 1,389 (1,697)
Balance at September 30, $ 1,820 $ 9,927 $ 6,217