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DERIVATIVE INSTRUMENTS - Narrative (Details) MMBTU in Millions, $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2021 USD ($) MW | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) MW | Sep. 30, 2020 USD ($) | Dec. 31, 2020 MMBTU MW
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |
Percentage of costs or benefits borne under capacity-related contracts by United Illuminated Company |  |  | 20.00% |  |
Amount of power to be purchased under capacity-related contract (in MW) | MW | 675 |  | 675 |  | 675
Amount of natural gas to be purchased under futures contracts | MMBTU |  |  |  |  | 8.9
Gains (losses) on deferred as regulatory costs | $ | $ 0.7 | $ (2.6) | $ (9.5) | $ (20.6) |
Level 3 | Minimum |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |
Percentage of exit price premiums related to derivative contracts |  |  | 5.50% |  |
Level 3 | Maximum |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |
Percentage of exit price premiums related to derivative contracts |  |  | 9.80% |  |
Level 3 | Weighted Average |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |
Percentage of exit price premiums related to derivative contracts |  |  | 8.70% |  |
CL&P |  |  |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |  |  |
Percentage of costs or benefits borne under capacity-related contracts |  |  | 80.00% |  |

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