QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Data [Line Items]
Operating Revenues	$ 2,034,900	$ 2,271,400	$ 1,853,900	$ 2,288,000	$ 1,895,600	$ 1,988,500	$ 1,762,800	$ 2,105,100	$ 8,448,201	$ 7,751,952	$ 7,639,129
Operating Income	400,000	466,000	391,400	442,500	443,700	495,300	448,200	501,000	1,699,930	1,888,249	1,841,274
Net Income	233,200	291,300	244,600	271,400	239,400	262,200	232,600	261,300	1,040,519	995,515	949,821
Net Income Attributable to Common Shareholders	$ 231,300	$ 289,400	$ 242,800	$ 269,500	$ 237,400	$ 260,400	$ 230,700	$ 259,500	1,033,000	987,996	942,302
Basic and Diluted EPS (in dollars per share)	$ 0.73	$ 0.91	$ 0.76	$ 0.85	$ 0.75	$ 0.82	$ 0.73	$ 0.82
The Connecticut Light And Power Company
Quarterly Financial Data [Line Items]
Operating Revenues	$ 751,300	$ 865,000	$ 694,900	$ 785,000	$ 713,700	$ 774,800	$ 666,600	$ 732,300	3,096,174	2,887,359	2,805,955
Operating Income	142,800	172,700	163,100	157,200	155,200	177,100	175,600	175,500	635,837	683,354	672,434
Net Income	79,100	100,300	99,700	98,600	99,100	96,100	91,300	90,200	377,717	376,726	334,254
NSTAR Electric Company
Quarterly Financial Data [Line Items]
Operating Revenues	712,600	939,500	690,700	770,100	690,200	851,900	704,700	733,800	3,112,926	2,980,629	3,041,588
Operating Income	126,000	205,500	133,600	119,000	124,200	229,700	177,900	156,600	584,123	688,440	663,731
Net Income	77,500	140,600	87,900	77,100	70,500	125,800	95,000	83,400	383,090	374,726	350,777
Public Service Company Of New Hampshire
Quarterly Financial Data [Line Items]
Operating Revenues	254,900	290,200	235,100	267,400	248,000	250,000	230,400	253,200	1,047,619	981,624	959,482
Operating Income	37,200	56,500	46,900	55,800	69,800	66,000	63,500	66,600	196,414	265,873	256,826
Net Income	$ 14,300	$ 40,700	$ 25,800	$ 35,100	$ 36,400	$ 33,700	$ 31,600	$ 34,300	$ 115,876	$ 135,996	$ 131,985