QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Data [Line Items]
Operating Revenues	$ 1,895,600	$ 1,988,500	$ 1,762,800	$ 2,105,100	$ 1,776,600	$ 2,039,700	$ 1,767,200	$ 2,055,600	$ 7,751,952	$ 7,639,129	$ 7,954,827
Operating Income	450,900	502,600	455,700	509,000	438,100	509,900	423,400	488,500	1,918,154	1,859,859	1,764,164
Net Income	239,400	262,200	232,600	261,300	231,100	267,200	205,500	246,000	995,515	949,821	886,004
Net Income Attributable to Common Shareholders	$ 237,400	$ 260,400	$ 230,700	$ 259,500	$ 229,200	$ 265,300	$ 203,600	$ 244,200	$ 987,996	$ 942,302	$ 878,485
Basic EPS (in dollars per share)	$ 0.75	$ 0.82	$ 0.73	$ 0.82	$ 0.72	$ 0.83	$ 0.64	$ 0.77	$ 3.11	$ 2.97	$ 2.77
Diluted EPS (in dollars per share)	$ 0.75	$ 0.82	$ 0.73	$ 0.82	$ 0.72	$ 0.83	$ 0.64	$ 0.77	$ 3.11	$ 2.96	$ 2.76
The Connecticut Light And Power Company
Quarterly Financial Data [Line Items]
Operating Revenues	$ 713,700	$ 774,800	$ 666,600	$ 732,300	$ 630,900	$ 760,000	$ 679,800	$ 735,300	$ 2,887,359	$ 2,805,955	$ 2,802,675
Operating Income	155,600	177,500	176,000	176,000	163,500	176,100	162,100	171,500	685,103	673,175	611,061
Net Income	99,100	96,100	91,300	90,200	77,800	86,600	82,900	87,000	376,726	334,254	299,360
NSTAR Electric Company
Quarterly Financial Data [Line Items]
Operating Revenues	690,200	851,900	704,700	733,800	687,400	904,400	707,600	742,200	2,980,629	3,041,588	3,198,887
Operating Income	128,900	234,400	182,700	161,600	130,800	240,800	159,700	142,900	707,627	674,164	758,347
Net Income	70,500	125,800	95,000	83,400	64,900	133,200	81,400	71,300	374,726	350,777	401,048
Public Service Company Of New Hampshire
Quarterly Financial Data [Line Items]
Operating Revenues	248,000	250,000	230,400	253,200	231,800	266,900	218,500	242,300	981,624	959,482	972,203
Operating Income	71,200	67,400	64,900	68,300	54,600	74,700	63,100	70,700	271,798	263,060	230,177
Net Income	$ 36,400	$ 33,700	$ 31,600	$ 34,300	$ 26,100	$ 38,500	$ 31,300	$ 36,100	$ 135,996	$ 131,985	$ 114,442