QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating Revenues	$ 1,068,000	$ 1,114,900	$ 1,047,500	$ 1,235,300	$ 1,204,100	$ 1,243,300	$ 1,111,400	$ 1,339,400	$ 4,465,657	$ 4,898,167	$ 5,439,430
Operating Income	184,900	203,800	178,100	227,400	195,300	199,600	178,300	226,700	794,176	799,891	751,375
Net Income	114,800	91,400	78,700	115,600	131,300	101,900	73,300	87,600	400,513	394,107	335,592
Net Income Attributable to Controlling Interests	113,200	90,000	77,300	114,200	129,300	100,500	71,900	86,200	394,693	387,949	330,033
Basic Earnings Per Common Share									$ 2.22	$ 2.20	$ 1.91
Diluted Earnings Per Common Share									$ 2.22	$ 2.19	$ 1.91
Earnings Per Share Basic And Diluted	$ 0.64	$ 0.51	$ 0.44	$ 0.64	$ 0.73	$ 0.57	$ 0.41	$ 0.49
The Connecticut Light And Power Company [Member]
Operating Revenues	593,000	673,700	608,000	673,700	707,000	789,200	707,900	795,000	2,548,387	2,999,102	3,424,538
Operating Income	84,700	137,700	114,800	126,000	124,600	131,400	106,200	125,500	463,183	487,650	465,106
Net Income	66,800	66,500	52,600	64,300	82,600	69,000	44,100	48,400	250,164	244,143	216,316
Public Service Company Of New Hampshire [Member]
Operating Revenues	243,700	259,600	240,200	269,500	259,500	277,000	238,300	258,600	1,013,003	1,033,439	1,109,591
Operating Income	46,800	48,500	37,900	46,900	43,100	49,800	43,400	39,900	180,104	176,186	134,587
Net Income	25,500	25,600	21,700	27,500	23,900	28,800	21,600	15,800	100,267	90,067	65,570
Western Massachusetts Electric Company [Member]
Operating Revenues	107,700	104,500	98,400	106,700	98,800	103,700	92,500	100,200	417,315	395,161	402,413
Operating Income	29,000	19,800	18,500	21,100	13,100	14,900	14,300	16,400	88,363	58,657	58,581
Net Income	$ 16,500	$ 8,400	$ 8,200	$ 10,000	$ 4,900	$ 7,300	$ 5,200	$ 5,700	$ 43,054	$ 23,090	$ 26,196
Quantifying Prior Year Misstatements Corrected in Current Year Financial Statements [Abstract]
Immaterial Error Correction									Distribution segment Net Income for the quarter ended September 30, 2011 decreased by $3.2 million, as compared to the quarter ended September 30, 2010, related to a pre-tax charge to establish a reserve of $5.3 million to reflect a wholesale billing adjustment, $4.3 million of which related to prior period amounts.