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SHORT-TERM DEBT - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 18, 2019
Oct. 25, 2019
Dec. 08, 2017
Commercial Paper          
Short-term Debt [Line Items]          
Amount of commercial paper program $ 346,300,000 $ 0      
Line of Credit          
Short-term Debt [Line Items]          
Amount outstanding during period 0 0      
Eversource | Commercial Paper          
Short-term Debt [Line Items]          
Amount of commercial paper program 1,450,000,000       $ 1,450,000,000
Eversource | Line of Credit          
Short-term Debt [Line Items]          
Amount of commercial paper program $ 1,450,000,000        
Debt instrument term 5 years        
CL&P          
Short-term Debt [Line Items]          
Short-term borrowing limit approved by regulatory agency       $ 600,000,000  
Notes payable to related parties $ 63,800,000 0      
NSTAR Electric          
Short-term Debt [Line Items]          
Short-term borrowing limit approved by regulatory agency     $ 655,000,000    
Notes payable to related parties 30,300,000 0      
NSTAR Electric | Commercial Paper          
Short-term Debt [Line Items]          
Amount of commercial paper program 650,000,000        
NSTAR Electric | Line of Credit          
Short-term Debt [Line Items]          
Amount of commercial paper program $ 650,000,000        
Debt instrument term 5 years        
PSNH          
Short-term Debt [Line Items]          
Short-term borrowing limit approved by regulatory agency $ 354,000,000        
Notes payable to related parties $ 27,000,000 $ 57,000,000      
PSNH | Short Term Debt Authorization Calculation          
Short-term Debt [Line Items]          
Interest rate stated percentage (up to) (as a percentage) 10.00%        
Short-term debt borrowing calculation approved by regulatory agency $ 60,000,000        
Unsecured Debt | CL&P          
Short-term Debt [Line Items]          
Period of maturity restricting provisions of debt 10 years        
Total capitalization (as a percentage) 10.00%        
Capacity available under preferred stock provisions $ 738,100,000