COMMON SHARES (Details) (USD $)	0 Months Ended
	Apr. 09, 2012	Mar. 31, 2013	Dec. 31, 2012
Class Of Stock [Line Items]
Common Stock Par Or Stated Value Per Share		$ 5	$ 5
Common Stock Shares Authorized		380,000,000	380,000,000
Common Stock Shares Issued		332,951,956	332,509,383
Treasury Stock Shares		18,356,487	18,455,749
Common Stock Issuance Merger With Nstar [Abstract]
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	136,000,000
The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Common Stock Par Or Stated Value Per Share		$ 10	$ 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	$ 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	$ 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	$ 25
Common Stock Shares Authorized		1,072,471	1,072,471
Common Stock Shares Issued		434,653	434,653