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EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Pension |  |  |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | $ (7,537) | $ (8,827) | $ (7,281)
Prior service (cost) credit | (106) | (111) | (111)
OPEB |  |  |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (4,660) | (4,370) | (3,274)
Prior service (cost) credit | 365 | 365 | 365
Regulatory Assets | Pension |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Regulatory Assets, Balance at beginning of period | 78,605 | 94,941 |
Amounts arising during the period, Net actuarial loss (gain) | (6,090) | (9,429) |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (6,177) | (6,799) |
Prior service (cost) credit | (105) | (108) |
Regulatory Assets, Balance at end of period | 66,233 | 78,605 | 94,941
Regulatory Assets | OPEB |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Regulatory Assets, Balance at beginning of period | 60,460 | 59,147 |
Amounts arising during the period, Net actuarial loss (gain) | 12,378 | 5,211 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (4,464) | (4,209) |
Prior service (cost) credit | 311 | 311 |
Regulatory Assets, Balance at end of period | 68,685 | 60,460 | 59,147
Accumulated Other Comprehensive Income (Loss) | Pension |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period | 19,415 | 28,436 |
Amounts arising during the period, Net actuarial loss (gain) | (3,422) | (6,990) |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (1,359) | (2,028) |
Prior service (cost) credit | (1) | (3) |
Accumulated Other Comprehensive Income (Loss), Balance at end of period | 14,633 | 19,415 | 28,436
Accumulated Other Comprehensive Income (Loss) | OPEB |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period | 4,967 | 4,486 |
Amounts arising during the period, Net actuarial loss (gain) | 2,834 | 587 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (196) | (160) |
Prior service (cost) credit | 54 | 54 |
Accumulated Other Comprehensive Income (Loss), Balance at end of period | $ 7,659 | $ 4,967 | $ 4,486

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