EMPLOYEE BENEFIT PLANS - AMOUNTS NOT YET RECOGNIZED AS NET PERIODIC COST (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Regulatory Assets | Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	$ 65,664	$ 77,930
Prior service cost (credit)	569	675
Regulatory Assets, Total	66,233	78,605	$ 94,941
Regulatory Assets | OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	69,477	61,563
Prior service cost (credit)	(792)	(1,103)
Regulatory Assets, Total	68,685	60,460	59,147
Accumulated Other Comprehensive Income (Loss) | Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	14,633	19,414
Prior service cost (credit)	0	1
Accumulated Other Comprehensive Income (Loss), Total	14,633	19,415	28,436
Accumulated Other Comprehensive Income (Loss) | OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	7,750	5,113
Prior service cost (credit)	(91)	(146)
Accumulated Other Comprehensive Income (Loss), Total	$ 7,659	$ 4,967	$ 4,486