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LONG-TERM DEBT - Narrative (Details) - USD ($)
$ in Thousands
1 Months Ended
Dec. 31, 2019
Jan. 31, 2021
Dec. 31, 2020
Dec. 16, 2020
Dec. 14, 2020
Jul. 31, 2020
Jan. 27, 2020
Debt Instrument [Line Items]              
Long-term debt $ 13,770,828   $ 15,125,876        
Payment to DOE 29,000            
Interest included in payment to settle spent nuclear fuel obligation 8,600   8,700        
NSTAR Electric Company              
Debt Instrument [Line Items]              
Amount of long-term debt requested for issuance         $ 1,600,000    
Long-term debt 3,247,086   3,393,221        
Public Service Company of New Hampshire              
Debt Instrument [Line Items]              
Amount of long-term debt approved for issuance           $ 200,000  
Long-term debt 951,620   817,070        
NSTAR Gas              
Debt Instrument [Line Items]              
Amount of long-term debt approved for issuance             $ 270,000
Aquarion Water Company              
Debt Instrument [Line Items]              
Amount of long-term debt requested for issuance       $ 100,000      
Pre-1983 Spent Nuclear Fuel Obligation (CYAPC)              
Debt Instrument [Line Items]              
Long-term debt 11,600   11,700        
PSNH 4.050% Series Q First Mortgage Bond | Public Service Company of New Hampshire              
Debt Instrument [Line Items]              
Long-term debt 957,000   $ 1,107,000        
PSNH 4.050% Series Q First Mortgage Bond | Public Service Company of New Hampshire | 4.050% Series Q due 2021              
Debt Instrument [Line Items]              
Interest rate     4.05%        
Long-term debt $ 122,000   $ 122,000        
PSNH 4.050% Series Q First Mortgage Bond | Public Service Company of New Hampshire | Subsequent Event | 4.050% Series Q due 2021              
Debt Instrument [Line Items]              
Interest rate   4.05%          
Face amount of debt   $ 122,000