QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating Revenues	$ 1,777,700	$ 1,892,600	$ 1,635,900	$ 1,995,000	$ 1,684,000	$ 1,861,500	$ 1,628,700	$ 1,099,600	$ 7,301,204	$ 6,273,787	$ 4,465,657
Operating Income	360,600	399,300	350,600	418,900	331,400	412,900	159,500	214,400	1,529,435	1,118,206	794,176
Net Income	179,200	211,400	173,100	230,000	176,600	209,500	46,200	100,800	793,689	533,077	400,513
Net Income Attributable to Controlling Interests	177,400	209,500	171,000	228,100	174,700	207,600	44,300	99,300	786,007	525,945	394,693
Basic Earnings Per Common Share									$ 2.49	$ 1.90	$ 2.22
Diluted Earnings Per Common Share									$ 2.49	$ 1.89	$ 2.22
Earnings Per Share Basic And Diluted	$ 0.56	$ 0.66	$ 0.54	$ 0.72	$ 0.55	$ 0.66	$ 0.15	$ 0.56
The Connecticut Light And Power Company [Member]
Operating Revenues	600,500	648,400	569,300	624,100	595,200	658,100	562,100	592,000	2,442,341	2,407,449	2,548,387
Operating Income	119,200	133,900	136,800	149,700	135,000	139,700	40,400	111,900	539,576	426,989	463,183
Net Income	60,200	66,300	67,900	85,000	73,900	74,900	6,900	54,000	279,412	209,725	250,164
NSTAR Electric Company [Member]
Operating Revenues	576,900	753,900	570,400	592,300	516,200	693,700	534,600	556,500	2,493,479	2,300,997	2,403,053
Operating Income	109,200	192,000	112,500	94,500	70,900	194,100	93,900	22,500	508,156	381,416	486,173
Net Income	55,400	107,000	58,000	48,100	34,000	106,800	45,500	3,900	268,546	190,242	252,494
Public Service Company Of New Hampshire [Member]
Operating Revenues	226,900	218,600	216,100	273,800	233,000	256,900	255,100	243,000	935,402	988,013	1,013,003
Operating Income	56,200	56,600	54,300	58,100	51,400	61,300	47,000	45,400	225,219	205,095	180,104
Net Income	26,800	28,400	27,200	29,000	27,200	27,200	21,200	21,300	111,397	96,882	100,267
Western Massachusetts Electric Company [Member]
Operating Revenues	110,900	121,800	115,000	125,000	107,900	112,500	106,800	114,000	472,724	441,164	417,315
Operating Income	22,400	28,900	32,400	35,600	28,900	28,100	25,100	28,700	119,347	110,774	88,363
Net Income	$ 10,400	$ 15,000	$ 16,400	$ 18,600	$ 15,100	$ 14,100	$ 11,100	$ 14,200	$ 60,438	$ 54,503	$ 43,054