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REGULATORY ACCOUNTING - Narrative (Details)
$ in Millions
 12 Months Ended
Nov. 01, 2025
USD ($)
Dec. 23, 2024
USD ($)
Nov. 07, 2024
USD ($)
Oct. 30, 2024
USD ($)
Aug. 01, 2024
USD ($)
Jun. 11, 2024
USD ($)
Dec. 31, 2023
USD ($)
Dec. 26, 2023
USD ($)
Oct. 30, 2023
USD ($)
May 01, 2023
USD ($)
Dec. 31, 2024
USD ($)
storm
Dec. 31, 2023
USD ($)
Aug. 14, 2024
USD ($)
Mar. 28, 2024
USD ($)
May 31, 2018
USD ($)
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved             $ 241.7       $ 221.0 $ 241.7      
Deferred costs             82.1       92.5 82.1      
Regulatory assets             6,389.2       7,070.7 6,389.2      
FERC ROE Complaints                              
Regulatory Assets [Line Items]                              
Cumulative pre-tax reserves which include impact of refunds given to customers                     39.1 39.1      
Deferred Pre-Staging And Storm Restoration Costs                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     410.0        
Deferred Storm Restoration Costs                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     $ 2,100.0        
Goodwill-related                              
Regulatory Assets [Line Items]                              
Recovery period (in years)                     40 years        
Remaining amortization (in years)                     15 years        
Regulatory assets             264.1       $ 247.2 264.1      
PSNH                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved             1.2       4.5 1.2      
Equity return not recorded on the balance sheet             10.2       $ 22.5 10.2      
Public utilities, request rate, unrecovered costs           $ 247.0                  
Public utilities, requested rate, unrecovered costs, recovery period           5 years                  
Public utilities, annual rate adjustment term                     4 years        
Public utilities, commitment agreement term           4 years                  
Regulatory assets             963.3       $ 1,065.7 963.3      
PSNH | FERC ROE Complaints                              
Regulatory Assets [Line Items]                              
Cumulative pre-tax reserves which include impact of refunds given to customers                     3.1 3.1      
PSNH | Deferred Pre-Staging And Storm Restoration Costs                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     191.0        
PSNH | Deferred Storm Restoration Costs                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     440.0        
PSNH | Goodwill-related                              
Regulatory Assets [Line Items]                              
Regulatory assets             0.0       0.0 0.0      
PSNH | Base Rate Adjustment                              
Regulatory Assets [Line Items]                              
Rate increase (decrease) requested           $ 181.9                  
Rate increase approved         $ 61.2                    
PSNH | Operation, Maintenance And Vegetation Expenses                              
Regulatory Assets [Line Items]                              
Purchase price of regulatory assets                   $ 23.3          
Regulatory assets                   $ 16.9          
CL&P                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved             166.7       116.3 166.7      
Deferred costs             64.0       47.2 64.0      
Public utilities, approved rate, recovery of incurred and deferred program costs                         $ 44.4    
Public utilities, approved rate, expected program costs                         24.3    
Public utilities, approved rate, recovery of incurred and deferred program costs, carrying costs                         $ 5.4    
Public utilities, approved rate, recovery of incurred and deferred program costs, period                         20 months    
Public utilities, approved rate, expected program costs, period                         8 months    
Regulatory assets             2,143.2       2,325.5 2,143.2      
CL&P | SBC                              
Regulatory Assets [Line Items]                              
Public utilities, approved rate, recovery, net of under recoveries             86.2         86.2      
Public utilities, approved rate, recovery of expected net costs             254.4         254.4      
CL&P | NBFMCC                              
Regulatory Assets [Line Items]                              
Public utilities, approved rate, recovery, net of under recoveries             264.9         264.9      
Public utilities, approved rate, recovery of expected net costs             388.5         388.5      
CL&P | 2024 Innovative Energy Solutions                              
Regulatory Assets [Line Items]                              
Public utilities, approved rate, incremental program costs                         $ 3.5    
CL&P | Connecticut Green Bank                              
Regulatory Assets [Line Items]                              
Public utilities, approved rate, incremental program costs                         $ 1.5    
CL&P | FERC ROE Complaints                              
Regulatory Assets [Line Items]                              
Cumulative pre-tax reserves which include impact of refunds given to customers                     21.4 21.4      
CL&P | Deferred Pre-Staging And Storm Restoration Costs                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     134.0        
CL&P | Deferred Storm Restoration Costs                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     1,120.0        
CL&P | Deferred Storm Restoration Costs | Tropical Storm Isaias                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     232.0        
CL&P | Catastrophic Storm Costs                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     173.0     $ 634.0  
CL&P | Goodwill-related                              
Regulatory Assets [Line Items]                              
Regulatory assets             0.0       0.0 0.0      
NSTAR Electric                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved             21.9       41.1 21.9      
Deferred costs             7.3       $ 24.4 $ 7.3      
Historical average term of capital additions (in years)   5 years                   5 years      
Rate increase approved               $ 104.9              
Storms in a calendar year | storm                     8        
Regulatory assets             1,957.9       $ 2,107.1 $ 1,957.9      
NSTAR Electric | FERC ROE Complaints                              
Regulatory Assets [Line Items]                              
Cumulative pre-tax reserves which include impact of refunds given to customers                     14.6 14.6      
NSTAR Electric | Deferred Pre-Staging And Storm Restoration Costs                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     85.0        
NSTAR Electric | Deferred Storm Restoration Costs                              
Regulatory Assets [Line Items]                              
Amount of regulatory costs not yet approved                     551.0        
NSTAR Electric | Goodwill-related                              
Regulatory Assets [Line Items]                              
Regulatory assets             226.7       212.3 $ 226.7      
NSTAR Electric | Inflation Based Adjustment                              
Regulatory Assets [Line Items]                              
Rate increase approved   $ 35.3           50.6              
NSTAR Electric | K-Bar Adjustment                              
Regulatory Assets [Line Items]                              
Rate increase approved               $ 54.3              
NSTAR Electric | Base Rate Adjustment                              
Regulatory Assets [Line Items]                              
Rate increase approved   55.8                          
NSTAR Electric | Capital Additions Adjustment                              
Regulatory Assets [Line Items]                              
Rate increase approved   $ 20.5                          
NSTAR Gas                              
Regulatory Assets [Line Items]                              
Rate increase approved       $ 12.7         $ 25.4            
NSTAR Gas | Base Rate Adjustment                              
Regulatory Assets [Line Items]                              
Rate increase approved                 $ 15.5            
Yankee Gas                              
Regulatory Assets [Line Items]                              
Rate increase (decrease) requested                     209.0        
Yankee Gas | Base Rate Adjustment                              
Regulatory Assets [Line Items]                              
Rate increase (decrease) requested                     274.0        
Yankee Gas | Offsetting Adjustment                              
Regulatory Assets [Line Items]                              
Rate increase (decrease) requested                     65.0        
Yankee Gas | Rate Credit                              
Regulatory Assets [Line Items]                              
Rate increase (decrease) requested                     $ 37.4        
EGMA                              
Regulatory Assets [Line Items]                              
Rate increase approved     $ 76.8                        
EGMA | GSEP Investments                              
Regulatory Assets [Line Items]                              
Rate increase approved     8.8       34.0                
EGMA | Base Rate Adjustment                              
Regulatory Assets [Line Items]                              
Rate increase approved     $ 85.6       $ 147.8                
EGMA | Base Rate Adjustment | Subsequent Event                              
Regulatory Assets [Line Items]                              
Public utilities, return on rate base, remaining revenue requirement $ 62.2                            
Rate Reduction Bonds | PSNH                              
Regulatory Assets [Line Items]                              
Amount of securitized rate reduction bonds issued                             $ 635.7