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COMMON SHARES (Details) - $ / shares
Sep. 30, 2018
Dec. 31, 2017
Class of Stock [Line Items]    
Per Value (in dollars per share) $ 5  
Authorized (in shares) 380,000,000  
Issued (in shares) 333,878,402 333,878,402
Treasury stock (in shares) 16,992,594 16,992,594
Common stock outstanding (in shares) 316,885,808 316,885,808
The Connecticut Light and Power Company    
Class of Stock [Line Items]    
Per Value (in dollars per share) $ 10  
Authorized (in shares) 24,500,000  
Issued (in shares) 6,035,205 6,035,205
NSTAR Electric Company    
Class of Stock [Line Items]    
Per Value (in dollars per share) $ 1  
Authorized (in shares) 100,000,000  
Issued (in shares) 200 200
Public Service Company of New Hampshire    
Class of Stock [Line Items]    
Per Value (in dollars per share) $ 1  
Authorized (in shares) 100,000,000  
Issued (in shares) 301 301