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EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - OPEB - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2024 | Jan. 02, 2024 | Mar. 31, 2023 | Mar. 31, 2024 | Mar. 31, 2023 | Jan. 01, 2024 | Sep. 30, 2023
Amounts amortized to net periodic costs: |  |  |  |  |  |  |
Net actuarial loss | $ (1,255) |  | $ 0 | $ (1,751) | $ 0 |  |
Regulatory Assets |  |  |  |  |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |  |
Regulatory Assets, Balance at beginning of period |  | $ 30,046 |  | 30,046 |  |  |
Net actuarial loss |  | 12,706 |  |  |  |  |
Prior service cost |  | (61,244) |  |  |  | $ (61,244) | $ 0
Amounts amortized to net periodic costs: |  |  |  |  |  |  |
Net actuarial loss |  | (355) |  |  |  |  |
Regulatory Assets, Balance at end of period |  | (18,847) |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period |  | 5,498 |  | $ 5,498 |  |  |
Net actuarial loss |  | 7,027 |  |  |  |  |
Prior service cost |  | (20,039) |  |  |  | $ (20,039) | $ 0
Amounts amortized to net periodic costs: |  |  |  |  |  |  |
Net actuarial loss |  | (141) |  |  |  |  |
Accumulated Other Comprehensive Income (Loss), Balance at end of period |  | $ (7,655) |  |  |  |  |

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