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COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details) - USD ($) $ in Millions |  |  | 9 Months Ended | 36 Months Ended |
--- | --- | --- | --- | --- | ---
Apr. 29, 2016 | Oct. 16, 2014 | Sep. 30, 2021 | Oct. 15, 2014 | Dec. 31, 2020
Loss Contingencies [Line Items] |  |  |  |  |
Percentage of base ROE reasonably justified |  |  | 10.41% |  |
Percentage of incentive cap on total ROE |  |  | 13.08% |  |
FERC ROE First, Second and Third Complaints |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Base ROE subsequently authorized |  |  | 11.14% |  |
FERC ROE First, Second and Third Complaints | Minimum |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Percentage of maximum ROE for any incentive project |  |  |  | 11.14% |
FERC ROE First, Second and Third Complaints | Maximum |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Percentage of maximum ROE for any incentive project |  |  |  | 13.10% |
FERC ROE Complaints |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Period of complaint |  |  | 15 months |  |
Basis point change |  |  | 0.10% |  |
Estimate of possible loss for each 10 basis point change to base ROE |  |  | $ 3.0 |  |
FERC ROE Fourth Complaint |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Base ROE percentage of complaint period | 10.57% |  |  |  |
Maximum percentage cap of ROE for incentive projects | 11.74% |  |  |  |
FERC ROE First Complaint |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Base ROE subsequently authorized |  | 10.57% |  |  |
Percentage of maximum ROE for any incentive project |  | 11.74% |  |  |
Amounts associated with first complaint refunded |  |  | 38.9 |  |
FERC ROE First Complaint | CL&P |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Amounts associated with first complaint refunded |  |  | 22.4 |  |
FERC ROE First Complaint | NSTAR Electric |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Amounts associated with first complaint refunded |  |  | 13.7 |  |
FERC ROE First Complaint | PSNH |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Amounts associated with first complaint refunded |  |  | $ 2.8 |  |
FERC ROE Second Complaint |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Base ROE percentage of complaint period |  |  | 10.57% |  |
Percentage of maximum ROE for any incentive project |  |  | 11.74% |  |
Loss contingency, estimate of possible earnings impact |  |  | $ 39.1 |  | $ 39.1
FERC ROE Second Complaint | CL&P |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Loss contingency, estimate of possible earnings impact |  |  | 21.4 |  | 21.4
FERC ROE Second Complaint | NSTAR Electric |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Loss contingency, estimate of possible earnings impact |  |  | 14.6 |  | 14.6
FERC ROE Second Complaint | PSNH |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |
Loss contingency, estimate of possible earnings impact |  |  | $ 3.1 |  | $ 3.1

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