QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Revenues	$ 1,776,600	$ 2,039,700	$ 1,767,200	$ 2,055,600	$ 1,691,200	$ 1,933,100	$ 1,817,100	$ 2,513,400	$ 7,639,129	$ 7,954,827	$ 7,741,856
Operating Income	438,100	509,900	423,400	488,500	385,500	469,200	412,000	497,500	1,859,859	1,764,164	1,632,849
Net Income	231,100	267,200	205,500	246,000	183,700	237,800	209,400	255,100	949,821	886,004	827,065
Net Income Attributable to Common Shareholders	$ 229,200	$ 265,300	$ 203,600	$ 244,200	$ 181,800	$ 235,900	$ 207,500	$ 253,300	$ 942,302	$ 878,485	$ 819,546
Basic Earnings Per Common Share (in dollars per share)	$ 0.72	$ 0.83	$ 0.64	$ 0.77	$ 0.57	$ 0.74	$ 0.65	$ 0.80	$ 2.97	$ 2.77	$ 2.59
Diluted Earnings Per Common Share (in dollars per share)	$ 0.72	$ 0.83	$ 0.64	$ 0.77	$ 0.57	$ 0.74	$ 0.65	$ 0.80	$ 2.96	$ 2.76	$ 2.58
The Connecticut Light And Power Company [Member]
Operating Revenues	$ 630,900	$ 760,000	$ 679,800	$ 735,300	$ 626,900	$ 704,300	$ 666,600	$ 804,900	$ 2,805,955	$ 2,802,675	$ 2,692,582
Operating Income	163,500	176,100	162,100	171,500	154,200	161,100	154,000	141,800	673,175	611,061	555,250
Net Income	77,800	86,600	82,900	87,000	71,200	80,200	78,800	69,200	334,254	299,360	287,754
NSTAR Electric Company [Member]
Operating Revenues	571,900	780,500	591,300	614,200	546,600	750,700	617,200	766,800	2,557,878	2,681,342	2,536,677
Operating Income	104,800	208,700	130,500	109,800	117,700	214,200	151,400	159,500	553,787	642,827	578,456
Net Income	52,800	117,200	68,200	54,500	60,300	118,600	82,000	83,600	292,705	344,542	303,088
Public Service Company Of New Hampshire [Member]
Operating Revenues	231,800	266,900	218,500	242,300	211,100	234,400	241,900	284,800	959,482	972,203	959,500
Operating Income	54,600	74,700	63,100	70,700	49,300	63,600	54,100	63,200	263,060	230,177	229,383
Net Income	26,100	38,500	31,300	36,100	22,000	32,500	27,900	32,000	131,985	114,442	113,944
Western Massachusetts Electric Company [Member]
Operating Revenues	115,700	124,000	116,400	128,100	114,900	125,100	125,200	152,900	484,192	518,128	493,423
Operating Income	26,000	32,100	29,200	33,100	28,000	30,000	28,900	28,600	120,377	115,520	117,639
Net Income	$ 12,000	$ 16,000	$ 13,300	$ 16,800	$ 14,100	$ 15,000	$ 14,200	$ 13,200	$ 58,072	$ 56,506	$ 57,819