EMPLOYEE BENEFIT PLANS - AMOUNTS NOT YET RECOGNIZED AS NET PERIODIC COST (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Regulatory Assets | Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss (gain)	$ 24,577	$ 35,157
Prior service cost	61	164
Regulatory Assets, Total	24,638	35,321	$ 56,187
Regulatory Assets | OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss (gain)	30,046	20,110
Prior service cost	0	0
Regulatory Assets, Total	30,046	20,110	60,335
Accumulated Other Comprehensive Income (Loss) | Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss (gain)	742	5,003
Prior service cost	0	0
Accumulated Other Comprehensive Income (Loss), Total	742	5,003	22,790
Accumulated Other Comprehensive Income (Loss) | OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss (gain)	5,498	(6,822)
Prior service cost	0	0
Accumulated Other Comprehensive Income (Loss), Total	$ 5,498	$ (6,822)	$ 12,696