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COMMON SHARES (Details) (USD $)
0 Months Ended
Apr. 09, 2012
Dec. 31, 2012
Dec. 31, 2011
Class Of Stock [Line Items]      
Common Stock Par Or Stated Value Per Share   $ 5  
Common Stock Shares Authorized   380,000,000 380,000,000
Common Stock Shares Issued   332,509,383 196,052,770
Treasury Stock Shares   18,455,749 18,894,078
Common Stock Issuance Merger With Nstar [Abstract]      
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares 136,049,000    
The Connecticut Light And Power Company [Member]
     
Class Of Stock [Line Items]      
Common Stock Par Or Stated Value Per Share   $ 10  
Common Stock Shares Authorized   24,500,000 24,500,000
Common Stock Shares Issued   6,035,205 6,035,205
NSTAR Electric Company [Member]
     
Class Of Stock [Line Items]      
Common Stock Par Or Stated Value Per Share   $ 1  
Common Stock Shares Authorized   100,000,000 100,000,000
Common Stock Shares Issued   100 100
Public Service Company Of New Hampshire [Member]
     
Class Of Stock [Line Items]      
Common Stock Par Or Stated Value Per Share   $ 1  
Common Stock Shares Authorized   100,000,000 100,000,000
Common Stock Shares Issued   301 301
Western Massachusetts Electric Company [Member]
     
Class Of Stock [Line Items]      
Common Stock Par Or Stated Value Per Share   $ 25  
Common Stock Shares Authorized   1,072,471 1,072,471
Common Stock Shares Issued   434,653 434,653