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COMMITMENTS AND CONTINGENCIES - CL&P Comprehensive Settlement Agreement (Details) - USD ($) $ / shares in Units, $ in Thousands |  | 9 Months Ended |  |
--- | --- | --- | --- | ---
Oct. 01, 2021 | Sep. 30, 2021 | Sep. 30, 2020 | Jul. 14, 2021 | Dec. 31, 2020
Loss Contingencies [Line Items] |  |  |  |  |
Regulatory Liabilities |  | $ 612,931 |  |  | $ 389,430
Loss related to litigation settlement |  | 103,583 | $ 0 |  |
CL&P |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Regulatory Liabilities |  | 311,269 |  |  | $ 137,166
Loss related to litigation settlement |  | 103,583 | $ 0 |  |
Subsequent Event |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Basis points reduction to return on equity, scenario one | 0.90% |  |  |  |
Basis points reduction to return on equity, scenario two | 0.45% |  |  |  |
Unfavorable Regulatory Action |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Loss related to litigation settlement |  | 103,600 |  |  |
Loss related to litigation settlement, net of tax |  | $ 85,800 |  |  |
Loss contingency, after-tax earnings impact per share (in dollars per share) |  | $ 0.25 |  | $ (0.07) |
Unfavorable Regulatory Action | Sales |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Loss related to litigation settlement |  | $ 65,000 |  |  |
Unfavorable Regulatory Action | Oil and Gas, Operation and Maintenance |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Loss related to litigation settlement |  | 10,000 |  |  |
Unfavorable Regulatory Action | CL&P |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Regulatory Liabilities |  | $ 75,000 |  |  |
Unfavorable Regulatory Action | Subsequent Event |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Customer credits | $ 65,000 |  |  |  |
Customer assistance initiatives | 10,000 |  |  |  |
Customer credits and assistance | $ 75,000 |  |  |  |

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