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COMMITMENTS AND CONTINGENT LIABILITIES - LEGAL PROCEEDINGS (Details) |  | 12 Months Ended |
--- | --- | --- | ---
Apr. 01, 2018 USD ($) | Sep. 30, 2017 USD ($) site | Sep. 30, 2016 USD ($)
Loss Contingencies [Line Items] |  |  |
Number of manufactured gas plant sites (in sites) | site |  | 5 |
Recovery period |  | 7 years |
Regulatory assets |  | $ 375,919,000 | $ 441,294,000
Minimum |  |  |
Loss Contingencies [Line Items] |  |  |
Litigation settlement, gross |  | 117,600,000 |
Loss contingency, estimate of possible loss |  | 600,000 |
Maximum |  |  |
Loss Contingencies [Line Items] |  |  |
Litigation settlement, gross |  | 205,200,000 |
Loss contingency, estimate of possible loss |  | 3,200,000.0 |
Environmental remediation costs, Expended, net of recoveries |  |  |
Loss Contingencies [Line Items] |  |  |
Regulatory assets |  | 28,547,000 | 19,595,000
Environmental remediation costs, Liability for future expenditures |  |  |
Loss Contingencies [Line Items] |  |  |
Regulatory assets |  | 149,000,000 | $ 172,000,000
RAC |  |  |
Loss Contingencies [Line Items] |  |  |
Approved rate, amount |  | $ 9,400,000 |
Scenario, Forecast | RAC |  |  |
Loss Contingencies [Line Items] |  |  |
Approved rate, amount | $ 7,000,000 |  |

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