QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Line Items]
Operating Revenues	$ 2,050,400	$ 2,175,800	$ 1,884,500	$ 2,415,800	$ 2,034,900	$ 2,271,400	$ 1,853,900	$ 2,288,000	$ 8,526,470	$ 8,448,201	$ 7,751,952
Operating Income	435,600	509,200	151,000	494,700	400,000	466,000	391,400	442,500	1,590,491	1,699,930	1,888,249
Net Income	251,900	320,800	33,300	310,600	233,200	291,300	244,600	271,400	916,572	1,040,519	995,515
Net Income Attributable to Common Shareholders	$ 250,000	$ 318,900	$ 31,500	$ 308,700	$ 231,300	$ 289,400	$ 242,800	$ 269,500	$ 909,053	$ 1,033,000	$ 987,996
Basic EPS (in dollars per share)	$ 0.77	$ 0.98	$ 0.10	$ 0.97	$ 0.73	$ 0.91	$ 0.76	$ 0.85	$ 2.83	$ 3.25	$ 3.11
Diluted EPS (in dollars per share)	$ 0.76	$ 0.98	$ 0.10	$ 0.97	$ 0.73	$ 0.91	$ 0.76	$ 0.85	$ 2.81	$ 3.25	$ 3.11
Impairment of Northern Pass Transmission			$ 239,600						$ 239,644	$ 0	$ 0
CL&P
Quarterly Financial Data [Line Items]
Operating Revenues	$ 788,700	$ 853,900	740,800	$ 849,200	$ 751,300	$ 865,000	$ 694,900	$ 785,000	3,232,551	3,096,174	2,887,359
Operating Income	152,700	190,300	166,800	171,800	142,800	172,700	163,100	157,200	681,649	635,837	683,354
Net Income	83,900	111,700	104,800	110,500	79,100	100,300	99,700	98,600	410,852	377,717	376,726
NSTAR Electric
Quarterly Financial Data [Line Items]
Operating Revenues	686,400	878,700	681,900	797,600	712,600	939,500	690,700	770,100	3,044,642	3,112,926	2,980,629
Operating Income	135,500	219,400	134,200	137,800	126,000	205,500	133,600	119,000	626,890	584,123	688,440
Net Income	93,400	154,900	89,700	94,000	77,500	140,600	87,900	77,100	431,956	383,090	374,726
PSNH
Quarterly Financial Data [Line Items]
Operating Revenues	268,200	280,400	240,900	276,400	254,900	290,200	235,100	267,400	1,065,936	1,047,619	981,624
Operating Income	56,000	64,500	46,300	49,700	37,200	56,500	46,900	55,800	216,467	196,414	265,873
Net Income	$ 33,400	$ 40,900	$ 26,900	$ 32,800	$ 14,300	$ 40,700	$ 25,800	$ 35,100	$ 134,048	$ 115,876	$ 135,996