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EMPLOYEE BENEFIT PLANS - AMOUNTS NOT YET RECOGNIZED AS NET PERIODIC COST (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
--- | --- | --- | ---
Regulatory Assets | Pension |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Net actuarial loss | $ 103,197 | $ 99,139 |
Prior service cost (credit) | 367 | 467 |
Regulatory Assets, Total | 103,564 | 99,606 | $ 66,233
Regulatory Assets | OPEB |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Net actuarial loss | 83,600 | 112,109 |
Prior service cost (credit) | (299) | (480) |
Regulatory Assets, Total | 83,301 | 111,629 | 68,685
Accumulated Other Comprehensive Income (Loss) | Pension |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Net actuarial loss | 33,004 | 27,801 |
Prior service cost (credit) | 0 | 0 |
Accumulated Other Comprehensive Income (Loss), Total | 33,004 | 27,801 | 14,633
Accumulated Other Comprehensive Income (Loss) | OPEB |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Net actuarial loss | 13,847 | 16,367 |
Prior service cost (credit) | (24) | (39) |
Accumulated Other Comprehensive Income (Loss), Total | $ 13,823 | $ 16,328 | $ 7,659

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