QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Revenues	$ 1,881,200	$ 1,892,500	$ 1,677,600	$ 2,290,600	$ 1,777,700	$ 1,892,600	$ 1,635,900	$ 1,995,000	$ 7,741,856	$ 7,301,204	$ 6,273,787
Operating Income	430,200	440,900	294,000	467,700	360,600	399,300	350,600	418,900	1,632,849	1,529,435	1,118,206
Net Income	223,600	236,500	129,200	237,800	179,200	211,400	173,100	230,000	827,065	793,689	533,077
Net Income Attributable to Controlling Interests	221,500	234,600	127,400	236,000	177,400	209,500	171,000	228,100	819,546	786,007	525,945
Basic Earnings Per Common Share									$ 2.59	$ 2.49	$ 1.90
Diluted Earnings Per Common Share									$ 2.58	$ 2.49	$ 1.89
Earnings Per Share Basic And Diluted	$ 0.69	$ 0.74	$ 0.40	$ 0.75	$ 0.56	$ 0.66	$ 0.54	$ 0.72
The Connecticut Light And Power Company [Member]
Operating Revenues	675,100	695,600	587,300	734,600	600,500	648,400	569,300	624,100	2,692,582	2,442,341	2,407,449
Operating Income	159,000	146,200	92,100	158,000	119,200	133,900	136,800	149,700	555,250	539,576	426,989
Net Income	87,200	83,900	37,400	79,300	60,200	66,300	67,900	85,000	287,754	279,412	209,725
NSTAR Electric Company [Member]
Operating Revenues	581,100	727,900	561,500	666,200	576,900	753,900	570,400	592,300	2,536,677	2,493,479	2,300,997
Operating Income	132,000	206,600	121,500	118,400	109,200	192,000	112,500	94,500	578,456	508,156	381,416
Net Income	69,300	115,600	60,100	58,100	55,400	107,000	58,000	48,100	303,088	268,546	190,242
Public Service Company Of New Hampshire [Member]
Operating Revenues	224,400	223,700	211,600	299,800	226,900	218,600	216,100	273,800	959,500	935,402	988,013
Operating Income	60,000	56,400	49,000	64,000	56,200	56,600	54,300	58,100	229,383	225,219	205,095
Net Income	29,000	28,200	24,100	32,600	26,800	28,400	27,200	29,000	113,944	111,397	96,882
Western Massachusetts Electric Company [Member]
Operating Revenues	129,600	118,100	108,300	137,400	110,900	121,800	115,000	125,000	493,423	472,724	441,164
Operating Income	34,000	31,200	17,700	34,700	22,400	28,900	32,400	35,600	117,639	119,347	110,774
Net Income	$ 18,000	$ 14,700	$ 7,000	$ 18,100	$ 10,400	$ 15,000	$ 16,400	$ 18,600	$ 57,819	$ 60,438	$ 54,503