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DERIVATIVE INSTRUMENTS - Derivative Contracts at Fair Value (Details) MMBTU in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) MMBTU MW | Dec. 31, 2019 USD ($)
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Percentage of costs or benefits borne under capacity-related contracts by United Illuminated Company (as a percentage) | 20.00% |  |
Amount of power to be purchased under capacity-related contract (up to) (in MW) | MW |  | 675 |
Amount of natural gas to be purchased under futures contracts (in MMBtu) | MMBTU |  | 8.9 |
Loss deferred as regulatory costs | $ | $ 7.1 | $ 21.2 | $ 20.7
The Connecticut Light and Power Company |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Percentage of costs or benefits borne under capacity-related contracts (as a percentage) | 80.00% |  |
Minimum | Level 3 |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Percentage of exit price premiums related to derivative contracts | 5.00% |  |
Minimum | Level 3 | The Connecticut Light and Power Company |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Percentage of exit price premiums related to derivative contracts | 9.30% |  |
Weighted Average | Level 3 |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Percentage of exit price premiums related to derivative contracts | 8.20% |  |

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- Definition Amount of Natural Gas To Be Purchased Under Futures Contracts + References No definition available. + Details
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