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FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Supplemental Fair Value Information about Long-Term Fixed-Rate Debt (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Sep. 30, 2019
Supplemental fair value information about long-term fixed-rate debt    
Long-term debt $ 480,727 $ 479,356
Carrying value    
Supplemental fair value information about long-term fixed-rate debt    
Long-term debt 480,700 479,400
Fair Value | Level 2    
Supplemental fair value information about long-term fixed-rate debt    
Fair value of long-term fixed-rate debt $ 534,500 $ 526,400