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REVENUES - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Sep. 01, 2021
May 06, 2021
Loss Contingencies [Line Items]          
Loss related to litigation settlement $ 81,274 $ 0 $ 0    
Revenues from Contracts with Customers 9,815,000 8,766,500 8,450,000    
Lease revenue 4,800 4,300 4,400    
The Connecticut Light and Power Company          
Loss Contingencies [Line Items]          
Loss related to litigation settlement 81,274 0 0    
Lease revenue 800 800 1,000    
NSTAR Electric Company          
Loss Contingencies [Line Items]          
Lease revenue 3,100 2,700 2,700    
Reserve for Revenues Subject to Refund          
Loss Contingencies [Line Items]          
Revenues from Contracts with Customers (78,700)        
Operating Segments | The Connecticut Light and Power Company          
Loss Contingencies [Line Items]          
Revenues from Contracts with Customers 4,238,900 4,044,700 3,626,900    
Operating Segments | NSTAR Electric Company          
Loss Contingencies [Line Items]          
Revenues from Contracts with Customers 3,541,600 3,327,500 3,396,400    
Operating Segments | Reserve for Revenues Subject to Refund | The Connecticut Light and Power Company          
Loss Contingencies [Line Items]          
Revenues from Contracts with Customers (76,100) 0 0    
Operating Segments | Reserve for Revenues Subject to Refund | NSTAR Electric Company          
Loss Contingencies [Line Items]          
Revenues from Contracts with Customers 0 $ 0 $ 0    
Unfavorable Regulatory Action          
Loss Contingencies [Line Items]          
Loss related to litigation settlement 103,600        
Unfavorable Regulatory Action | The Connecticut Light and Power Company          
Loss Contingencies [Line Items]          
Loss contingency, estimate of possible earnings impact, non-compliance with performance standards       $ 28,400 $ 28,400
Unfavorable Regulatory Action | Operating Segments | Reserve for Revenues Subject to Refund | The Connecticut Light and Power Company          
Loss Contingencies [Line Items]          
Revenues from Contracts with Customers 93,400        
Sales | Unfavorable Regulatory Action          
Loss Contingencies [Line Items]          
Loss related to litigation settlement 65,000        
Sales | Unfavorable Regulatory Action | The Connecticut Light and Power Company          
Loss Contingencies [Line Items]          
Loss related to litigation settlement $ 65,000