SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENTS OF INCOME (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
Condensed Income Statements, Captions [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 Expenses:
Other									1,363,113	1,335,213	1,307,052
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
Interest Expense									533,197	498,805	421,755
Other Income, Net:
Equity in Earnings of Subsidiaries									42,200	3,800	27,400
Other Income, Net									132,777	128,366	107,913
Income Before Income Tax Expense									1,190,071	1,329,491	1,574,407
Income Tax Benefit									273,499	288,972	578,892
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
Basic Earnings Per Common Share (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 Earnings Per Common Share (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
Weighted Average Common Shares Outstanding:
Basic (in shares)									321,416,086	317,370,369	317,411,097
Diluted (in shares)									322,941,636	317,993,934	318,031,580
Eversource
Condensed Income Statements, Captions [Line Items]
Operating Revenues									$ 0	$ 0	$ 0
Operating Expenses:
Other									50,100	(6,552)	(32,189)
Operating Income									(50,100)	6,552	32,189
Interest Expense									163,937	123,638	80,700
Other Income, Net:
Equity in Earnings of Subsidiaries									1,001,526	1,049,748	993,063
Other, Net									68,137	47,581	23,339
Other Income, Net									1,069,663	1,097,329	1,016,402
Income Before Income Tax Expense									855,626	980,243	967,891
Income Tax Benefit									(53,427)	(52,757)	(20,105)
Net Income									$ 909,053	$ 1,033,000	$ 987,996
Basic Earnings Per Common Share (in dollars per share)									$ 2.83	$ 3.25	$ 3.11
Diluted Earnings Per Common Share (in dollars per share)									$ 2.81	$ 3.25	$ 3.11
Weighted Average Common Shares Outstanding:
Basic (in shares)									321,416,086	317,370,369	317,411,097
Diluted (in shares)									322,941,636	317,993,934	318,031,580