EMPLOYEE BENEFIT PLANS, AMOUNTS NOT YET RECOGNIZED AS NET PERIODIC COST (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Regulatory Assets | Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	$ 94,158	$ 86,070
Prior service cost (credit)	783	890
Regulatory Assets, Total	94,941	86,960
Regulatory Assets | OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	60,561	52,462
Prior service cost (credit)	(1,414)	(1,725)
Regulatory Assets, Total	59,147	50,737
Accumulated Other Comprehensive Income (Loss) | Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	28,432	25,632
Prior service cost (credit)	4	8
Accumulated Other Comprehensive Income (Loss), Total	28,436	25,640
Accumulated Other Comprehensive Income (Loss) | OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	4,686	1,495
Prior service cost (credit)	(200)	(253)
Accumulated Other Comprehensive Income (Loss), Total	$ 4,486	$ 1,242