EMPLOYEE BENEFIT PLANS - AMOUNTS NOT YET RECOGNIZED AS NET PERIODIC COST (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Regulatory Assets | Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	$ 99,139	$ 65,664
Prior service cost (credit)	467	569
Regulatory Assets, Total	99,606	66,233	$ 78,605
Regulatory Assets | OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	112,109	69,477
Prior service cost (credit)	(480)	(792)
Regulatory Assets, Total	111,629	68,685	60,460
Accumulated Other Comprehensive Income (Loss) | Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	27,801	14,633
Prior service cost (credit)	0	0
Accumulated Other Comprehensive Income (Loss), Total	27,801	14,633	19,415
Accumulated Other Comprehensive Income (Loss) | OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	16,367	7,750
Prior service cost (credit)	(39)	(91)
Accumulated Other Comprehensive Income (Loss), Total	$ 16,328	$ 7,659	$ 4,967