SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENTS OF INCOME (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
Condensed Income Statements, Captions [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 Expenses:
Other									1,335,213	1,307,052	1,342,134
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
Interest Expense									498,805	421,755	400,961
Other Income, Net:
Other Income, Net									128,366	107,913	64,505
Income Before Income Tax Benefit									1,329,491	1,574,407	1,504,818
Income Tax Benefit									288,972	578,892	554,997
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
Basic Earnings Per Common Share (in dollars per share)									$ 3.25	$ 3.11	$ 2.97
Diluted Earnings Per Common Share (in dollars per share)									$ 3.25	$ 3.11	$ 2.96
Weighted Average Common Shares Outstanding:
Basic (in shares)									317,370,369	317,411,097	317,650,180
Diluted (in shares)									317,993,934	318,031,580	318,454,239
Eversource
Condensed Income Statements, Captions [Line Items]
Operating Revenues									$ 0	$ 0	$ 0
Operating Expenses:
Other									(6,552)	(32,189)	(39,453)
Operating Income									6,552	32,189	39,453
Interest Expense									123,638	80,700	59,420
Other Income, Net:
Equity in Earnings of Subsidiaries									1,049,748	993,063	922,321
Other, Net									47,581	23,339	4,267
Other Income, Net									1,097,329	1,016,402	926,588
Income Before Income Tax Benefit									980,243	967,891	906,621
Income Tax Benefit									(52,757)	(20,105)	(35,681)
Net Income									$ 1,033,000	$ 987,996	$ 942,302
Basic Earnings Per Common Share (in dollars per share)									$ 3.25	$ 3.11	$ 2.97
Diluted Earnings Per Common Share (in dollars per share)									$ 3.25	$ 3.11	$ 2.96
Weighted Average Common Shares Outstanding:
Basic (in shares)									317,370,369	317,411,097	317,650,180
Diluted (in shares)									317,993,934	318,031,580	318,454,239