SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENTS OF INCOME (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
Condensed Income Statements, Captions [Line Items]
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 Expenses:
Other									1,323,549	1,329,289	1,427,589
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
Interest Expense									400,961	372,420	362,106
Other Income, Net:
Other Income, Net									45,920	34,227	24,619
Income Before Income Tax Expense									1,504,818	1,425,971	1,295,362
Income Tax Benefit									554,997	539,967	468,297
Net Income	$ 231,100	$ 267,200	$ 205,500	$ 246,000	$ 183,700	$ 237,800	$ 209,400	$ 255,100	$ 949,821	$ 886,004	$ 827,065
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
Weighted Average Common Shares Outstanding:
Basic (in shares)									317,650,180	317,336,881	316,136,748
Diluted (in shares)									318,454,239	318,432,687	317,417,414
Eversource [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenues									$ 0	$ 0	$ 0
Operating Expenses:
Other									(39,453)	9,315	29,598
Operating Income									39,453	(9,315)	(29,598)
Interest Expense									59,420	45,130	33,168
Other Income, Net:
Equity in Earnings of Subsidiaries									922,321	900,824	848,435
Other, Net									4,267	6,602	1,830
Other Income, Net									926,588	907,426	850,265
Income Before Income Tax Expense									906,621	852,981	787,499
Income Tax Benefit									(35,681)	(25,504)	(32,047)
Net Income									$ 942,302	$ 878,485	$ 819,546