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EMPLOYEE BENEFIT PLANS - REGULATORY ASSETS AND AOCI (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Pension |  |  |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | $ (8,827) | $ (7,281) | $ (6,985)
Prior service (cost) credit | (111) | (111) | (111)
OPEB |  |  |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (4,370) | (3,274) | (2,943)
Prior service (cost) credit | 365 | 365 | 364
Regulatory Assets | Pension |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Regulatory Assets, Balance at beginning of period | 94,941 | 86,960 |
Amounts arising during the period, Net actuarial loss (gain) | (9,429) | 13,696 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (6,799) | (5,607) |
Prior service (cost) credit | (108) | (108) |
Regulatory Assets, Balance at end of period | 78,605 | 94,941 | 86,960
Regulatory Assets | OPEB |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Regulatory Assets, Balance at beginning of period | 59,147 | 50,737 |
Amounts arising during the period, Net actuarial loss (gain) | 5,211 | 11,274 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (4,209) | (3,175) |
Prior service (cost) credit | 311 | 311 |
Regulatory Assets, Balance at end of period | 60,460 | 59,147 | 50,737
Accumulated Other Comprehensive Income (Loss) | Pension |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period | 28,436 | 25,640 |
Amounts arising during the period, Net actuarial loss (gain) | (6,990) | 4,475 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (2,028) | (1,676) |
Prior service (cost) credit | (3) | (3) |
Accumulated Other Comprehensive Income (Loss), Balance at end of period | 19,415 | 28,436 | 25,640
Accumulated Other Comprehensive Income (Loss) | OPEB |  |  |
Regulatory Assets and Accumulated Other Comprehensive Income (Loss) |  |  |
Accumulated Other Comprehensive Income (Loss), Balance at beginning of period | 4,486 | 1,242 |
Amounts arising during the period, Net actuarial loss (gain) | 587 | 3,289 |
Amounts amortized to net periodic costs: |  |  |
Net actuarial (loss) | (160) | (99) |
Prior service (cost) credit | 54 | 54 |
Accumulated Other Comprehensive Income (Loss), Balance at end of period | $ 4,967 | $ 4,486 | $ 1,242

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