FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Supplemental Fair Value Information about Long-Term Fixed-Rate Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Supplemental fair value information about long-term fixed-rate debt
Carrying value of long-term fixed-rate debt	$ 480,269	$ 479,356
Fair value of long-term fixed-rate debt	520,500
Level 2
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt		526,400
Carrying Value
Supplemental fair value information about long-term fixed-rate debt
Carrying value of long-term fixed-rate debt	$ 480,300	$ 479,400