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DERIVATIVE INSTRUMENTS - Summary of Level 3 Derivative Contracts and Significant Unobservable Inputs Used (Details) - CL&P - Level 3 - $ / KWmo | Sep. 30, 2021 | Dec. 31, 2020
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Capacity Prices |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative measurement input | 2.61 |
Capacity Prices | Minimum |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative measurement input |  | 4.30
Capacity Prices | Maximum |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative measurement input |  | 5.30
Capacity Prices | Weighted Average |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative measurement input | 2.61 | 4.63
Forward Reserve Prices | Minimum |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative measurement input | 0.50 | 0.54
Forward Reserve Prices | Maximum |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative measurement input | 1.15 | 0.90
Forward Reserve Prices | Weighted Average |  |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |  |
Derivative measurement input | 0.82 | 0.72

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