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FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Supplemental Fair Value Information about Long-Term Fixed-Rate Debt (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Sep. 30, 2019
Supplemental fair value information about long-term fixed-rate debt    
Carrying value of long-term fixed-rate debt $ 479,811 $ 479,356
Level 2    
Supplemental fair value information about long-term fixed-rate debt    
Fair value of long-term fixed-rate debt 494,300 526,400
Carrying Value    
Supplemental fair value information about long-term fixed-rate debt    
Carrying value of long-term fixed-rate debt $ 479,800 $ 479,400