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COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details)
$ in Millions
3 Months Ended 36 Months Ended
Apr. 29, 2016
complaint
Oct. 16, 2014
Mar. 31, 2023
USD ($)
period
Oct. 15, 2014
Dec. 31, 2022
USD ($)
Dec. 23, 2019
complaint
Nov. 21, 2019
complaint
Oct. 16, 2018
complaint
Jul. 31, 2014
complaint
Loss Contingencies [Line Items]                  
Percentage of base ROE reasonably justified     10.41%            
Percentage of incentive cap on total ROE     13.08%            
FERC ROE Complaints                  
Loss Contingencies [Line Items]                  
Number of complaints filed | complaint 4         4 2 4  
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            
Number of complain periods | period     4            
FERC ROE First, Second and Third Complaints                  
Loss Contingencies [Line Items]                  
Number of complaints filed | complaint                 3
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 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