QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Revenues	$ 1,684,000	$ 1,861,500	$ 1,628,700	$ 1,099,600	$ 1,068,000	$ 1,114,900	$ 1,047,500	$ 1,235,300	$ 6,273,787	$ 4,465,657	$ 4,898,167
Operating Income	331,400	412,900	159,500	214,400	184,900	203,800	178,100	227,400	1,118,206	794,176	799,891
Net Income	176,600	209,500	46,200	100,800	114,800	91,400	78,700	115,600	533,077	400,513	394,107
Net Income Attributable to Controlling Interests	174,700	207,600	44,300	99,300	113,200	90,000	77,300	114,200	525,945	394,693	387,949
Basic Earnings Per Common Share									$ 1.90	$ 2.22	$ 2.20
Diluted Earnings Per Common Share									$ 1.89	$ 2.22	$ 2.19
Earnings Per Share Basic And Diluted	$ 0.55	$ 0.66	$ 0.15	$ 0.56	$ 0.64	$ 0.51	$ 0.44	$ 0.64
Quantifying Prior Year Misstatements Corrected in Current Year Financial Statements [Abstract]
Immaterial Error Correction									The NSTAR Electric consolidated statements of cash flows were revised to correct an error in the presentation of cash deposits related to the RRBs.The impact of this revision was an increase in investing cash inflows from Other Investing Activities in an amount of $1.7 million and $24.1 million and a corresponding increase to financing cash outflows from Retirements ofRate Reduction Bonds for the years ended December 31, 2011 and 2010, respectively. These revisions had no impact on NSTAR Electric’s results of operations or cash balance and are not deemed material, individually orin the aggregate, to the previously issued consolidated financial statements.
The Connecticut Light And Power Company [Member]
Operating Revenues	595,200	658,100	562,100	592,000	593,000	673,700	608,000	673,700	2,407,449	2,548,387	2,999,102
Operating Income	135,000	139,700	40,400	111,900	84,700	137,700	114,800	126,000	426,989	463,183	487,650
Net Income	73,900	74,900	6,900	54,000	66,800	66,500	52,600	64,300	209,725	250,164	244,143
Public Service Company Of New Hampshire [Member]
Operating Revenues	233,000	256,900	255,100	243,000	243,700	259,600	240,200	269,500	988,013	1,013,003	1,033,439
Operating Income	51,400	61,300	47,000	45,400	46,800	48,500	37,900	46,900	205,095	180,104	176,186
Net Income	27,200	27,200	21,200	21,300	25,500	25,600	21,700	27,500	96,882	100,267	90,067
Western Massachusetts Electric Company [Member]
Operating Revenues	107,900	112,500	106,800	114,000	107,700	104,500	98,400	106,700	441,164	417,315	395,161
Operating Income	28,900	28,100	25,100	28,700	29,000	19,800	18,500	21,100	110,774	88,363	58,657
Net Income	15,100	14,100	11,100	14,200	16,500	8,400	8,200	10,000	54,503	43,054	23,090
NSTAR Electric Company [Member]
Operating Revenues	516,200	693,700	534,600	556,500	548,600	724,700	552,300	577,500	2,300,997	2,403,053	2,366,201
Operating Income	70,900	194,100	93,900	22,500	99,200	183,700	115,600	87,700	381,416	486,173	478,812
Net Income	$ 34,000	$ 106,800	$ 45,500	$ 3,900	$ 49,100	$ 99,800	$ 60,700	$ 42,900	$ 190,242	$ 252,494	$ 248,575