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REGULATORY LIABILITIES (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2021 | Dec. 31, 2019 | Dec. 31, 2020 | Jan. 31, 2019
REGULATORY LIABILITIES [Abstract] |  |  |  |
Federal corporate tax rate | 21.00% | 34.00% |  |
Decrease in the net deferred income tax liability | $ 24,300 |  |  |
Regulatory Liabilities [Abstract] |  |  |  |
Regulatory liabilities | $ 21,361 | $ 22,800 | $ 21,681 |
DEPSC [Member] |  |  |  |
Regulatory Liabilities [Abstract] |  |  |  |
Regulatory liabilities |  |  |  | $ 22,200
Regulatory liabilities, amortization period | 49 years 6 months |  |  |
MDPSC [Member] |  |  |  |
Regulatory Liabilities [Abstract] |  |  |  |
Regulatory liabilities | $ 600 |  |  |
Utility Plant Retirement Cost Obligation [Member] |  |  |  |
Regulatory Liabilities [Abstract] |  |  |  |
Regulatory liabilities | 139 |  | 126 |
Deferred Income Taxes [Member] |  |  |  |
Regulatory Liabilities [Abstract] |  |  |  |
Regulatory liabilities | $ 21,222 |  | $ 21,555 |

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