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DERIVATIVE INSTRUMENTS - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2023 USD ($) MW | Dec. 31, 2022 USD ($) $ / KWmo MW | Dec. 31, 2021 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 | 682 | 674 |
Gain (loss) on deferred regulatory costs | $ | $ (3.9) | $ 10.1 | $ (7.1)
CL&P |  |  |
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 |  | 2.90% |
Minimum | Level 3 | CL&P | Measurement Input, Forward Reserve |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative measurement input |  | 0.44 |
Weighted Average | Level 3 |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Percentage of exit price premiums related to derivative contracts |  | 6.10% |
Weighted Average | Level 3 | CL&P | Measurement Input, Forward Reserve |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative measurement input |  | 0.47 |
Maximum | Level 3 |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Percentage of exit price premiums related to derivative contracts |  | 7.10% |
Maximum | Level 3 | CL&P | Measurement Input, Forward Reserve |  |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |  |
Derivative measurement input |  | 0.50 |

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- Definition Percentage of Costs or Benefits Borne Under Capacity-Related Contracts by United Illuminated Company + References No definition available. + Details
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- Definition Value of input used to measure net derivative asset (liability). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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