SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENTS OF INCOME (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
Condensed Income Statements, Captions [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 Expenses:
Other									1,277,147	1,323,549	1,329,289
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
Interest Expense									421,755	400,961	372,420
Other Income, Net:
Other Income, Net									78,008	45,920	34,227
Income Before Income Tax Expense									1,574,407	1,504,818	1,425,971
Income Tax Benefit									578,892	554,997	539,967
Net Income	$ 239,400	$ 262,200	$ 232,600	$ 261,300	$ 231,100	$ 267,200	$ 205,500	$ 246,000	$ 995,515	$ 949,821	$ 886,004
Basic Earnings Per Common Share (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 Earnings Per Common Share (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
Weighted Average Common Shares Outstanding:
Basic (in shares)									317,411,097	317,650,180	317,336,881
Diluted (in shares)									318,031,580	318,454,239	318,432,687
Eversource
Condensed Income Statements, Captions [Line Items]
Operating Revenues									$ 0	$ 0	$ 0
Operating Expenses:
Other									(32,189)	(39,453)	9,315
Operating Income									32,189	39,453	(9,315)
Interest Expense									80,700	59,420	45,130
Other Income, Net:
Equity in Earnings of Subsidiaries									993,063	922,321	900,824
Other, Net									23,339	4,267	6,602
Other Income, Net									1,016,402	926,588	907,426
Income Before Income Tax Expense									967,891	906,621	852,981
Income Tax Benefit									(20,105)	(35,681)	(25,504)
Net Income									$ 987,996	$ 942,302	$ 878,485
Basic Earnings Per Common Share (in dollars per share)									$ 3.11	$ 2.97	$ 2.77
Diluted Earnings Per Common Share (in dollars per share)									$ 3.11	$ 2.96	$ 2.76
Weighted Average Common Shares Outstanding:
Basic (in shares)									317,411,097	317,650,180	317,336,881
Diluted (in shares)									318,031,580	318,454,239	318,432,687