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EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Pension |  |  |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | $ (10,424) | $ (5,765) | $ (7,537)
Prior service credit | (102) | (102) | (106)
OPEB |  |  |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (7,442) | (6,466) | (4,660)
Prior service credit | 197 | 365 | 365
Regulatory Assets | Pension |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Regulatory Assets, Balance at beginning of period | 99,606 | 66,233 |
Amounts arising during the period, Net actuarial loss (gain) | 11,953 | 38,137 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (7,893) | (4,662) |
Prior service credit | (102) | (102) |
Regulatory Assets, Balance at end of period | 103,564 | 99,606 | 66,233
Regulatory Assets | OPEB |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Regulatory Assets, Balance at beginning of period | 111,629 | 68,685 |
Amounts arising during the period, Net actuarial loss (gain) | (21,974) | 48,452 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (6,536) | (5,820) |
Prior service credit | 182 | 312 |
Regulatory Assets, Balance at end of period | 83,301 | 111,629 | 68,685
Accumulated Other Comprehensive Income (Loss) | Pension |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period | 27,801 | 14,633 |
Amounts arising during the period, Net actuarial loss (gain) | 7,731 | 14,271 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (2,528) | (1,103) |
Prior service credit | 0 | 0 |
Accumulated Other Comprehensive Income (Loss), Balance at end of period | 33,004 | 27,801 | 14,633
Accumulated Other Comprehensive Income (Loss) | OPEB |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period | 16,328 | 7,659 |
Amounts arising during the period, Net actuarial loss (gain) | (1,614) | 9,264 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (907) | (648) |
Prior service credit | 16 | 53 |
Accumulated Other Comprehensive Income (Loss), Balance at end of period | $ 13,823 | $ 16,328 | $ 7,659

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