QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Revenues	$ 1,691,200	$ 1,933,100	$ 1,817,100	$ 2,513,400	$ 1,881,200	$ 1,892,500	$ 1,677,600	$ 2,290,600	$ 7,954,827	$ 7,741,856	$ 7,301,204
Operating Income	385,500	469,200	412,000	497,500	430,200	440,900	294,000	467,700	1,764,164	1,632,849	1,529,435
Net Income	183,700	237,800	209,400	255,100	223,600	236,500	129,200	237,800	886,004	827,065	793,689
Net Income Attributable to Controlling Interests	$ 181,800	$ 235,900	$ 207,500	$ 253,300	$ 221,500	$ 234,600	$ 127,400	$ 236,000	$ 878,485	$ 819,546	$ 786,007
Basic Earnings Per Common Share									$ 2.77	$ 2.59	$ 2.49
Diluted Earnings Per Common Share									$ 2.76	$ 2.58	$ 2.49
Earnings Per Share Basic And Diluted	$ 0.57	$ 0.74	$ 0.65	$ 0.80	$ 0.69	$ 0.74	$ 0.40	$ 0.75
The Connecticut Light And Power Company [Member]
Operating Revenues	$ 626,900	$ 704,300	$ 666,600	$ 804,900	$ 675,100	$ 695,600	$ 587,300	$ 734,600	$ 2,802,675	$ 2,692,582	$ 2,442,341
Operating Income	154,200	161,100	154,000	141,800	159,000	146,200	92,100	158,000	611,061	555,250	539,576
Net Income	71,200	80,200	78,800	69,200	87,200	83,900	37,400	79,300	299,360	287,754	279,412
NSTAR Electric Company [Member]
Operating Revenues	546,600	750,700	617,200	766,800	581,100	727,900	561,500	666,200	2,681,342	2,536,677	2,493,479
Operating Income	117,700	214,200	151,400	159,500	132,000	206,600	121,500	118,400	642,827	578,456	508,156
Net Income	60,300	118,600	82,000	83,600	69,300	115,600	60,100	58,100	344,542	303,088	268,546
Public Service Company Of New Hampshire [Member]
Operating Revenues	211,100	234,400	241,900	284,800	224,400	223,700	211,600	299,800	972,203	959,500	935,402
Operating Income	49,300	63,600	54,100	63,200	60,000	56,400	49,000	64,000	230,177	229,383	225,219
Net Income	22,000	32,500	27,900	32,000	29,000	28,200	24,100	32,600	114,442	113,944	111,397
Western Massachusetts Electric Company [Member]
Operating Revenues	114,900	125,100	125,200	152,900	129,600	118,100	108,300	137,400	518,128	493,423	472,724
Operating Income	28,000	30,000	28,900	28,600	34,000	31,200	17,700	34,700	115,520	117,639	119,347
Net Income	$ 14,100	$ 15,000	$ 14,200	$ 13,200	$ 18,000	$ 14,700	$ 7,000	$ 18,100	$ 56,506	$ 57,819	$ 60,438