SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENT OF CASH FLOW (Details) - USD ($) $ in Thousands				4 Months Ended	7 Months Ended	12 Months Ended
	Jun. 15, 2020	Mar. 26, 2020	Mar. 23, 2020	Jun. 15, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities:
Net Income						$ 1,212,686	$ 916,572	$ 1,040,519
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Equity in Earnings of Subsidiaries						(14,200)	(42,200)	(3,800)
Deferred Income Taxes						257,154	209,812	174,812
Other						(174,289)	(196,087)	(111,225)
Accounts Receivables from Subsidiaries						(351,843)	(98,716)	(141,433)
Taxes Receivable/Accrued, Net						43,819	(16,129)	(67,770)
Other Current Assets and Liabilities, Net						(9,591)	(11,603)	78,226
Net Cash Flows Provided by Operating Activities						1,682,572	2,009,577	1,830,543
Investing Activities:
Other Investing Activities						23,809	24,204	6,754
Net Cash Flows Used in Investing Activities						(4,129,275)	(3,274,288)	(2,437,046)
Financing Activities:
Issuance of Common Shares, Net of Issuance Costs	$ 509,200	$ 314,100	$ 105,700	$ 929,000	$ 852,300	928,992	852,254	0
Cash Dividends on Common Shares						(744,665)	(663,239)	(640,110)
Issuance of Long-Term Debt						2,760,000	1,520,000	2,200,000
Retirement of Long-Term Debt						(327,236)	(801,078)	(1,050,330)
Decrease in Notes Payable								(379,310)
Other Financing Activities						14,273	(1,006)	(28,457)
Net Cash Flows Provided by/(Used in) Financing Activities						2,594,590	1,172,450	729,937
Cash, Cash Equivalents and Restricted Cash - Beginning of Year						15,432
Cash, Cash Equivalents and Restricted Cash - End of Year					15,432	106,599	15,432
Cash Paid/(Received) During the Year for:
Interest						518,000	532,400	503,200
Income Taxes						48,900	56,000	158,800
Eversource
Operating Activities:
Net Income						1,205,167	909,053	1,033,000
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Equity in Earnings of Subsidiaries						(1,309,630)	(1,001,526)	(1,049,748)
Cash Dividends Received from Subsidiaries						485,800	883,000	569,500
Deferred Income Taxes						(4,667)	13,382	20,032
Other						39,940	19,584	(31,093)
Accounts Receivables from Subsidiaries						(14,575)	7,105	(28,716)
Taxes Receivable/Accrued, Net						35,300	(605)	(20,207)
Accounts Payable to Subsidiaries						14,091	(4,099)	(9,817)
Other Current Assets and Liabilities, Net						21,284	(2,503)	2,553
Net Cash Flows Provided by Operating Activities						472,710	823,391	485,504
Investing Activities:
Capital Contributions to Subsidiaries						(1,899,340)	(1,039,000)	(955,700)
Return of Capital from Subsidiaries						80,000	0	530,000
Increase in Notes Receivable from Subsidiaries						(264,300)	(218,100)	(158,210)
Other Investing Activities						(367)	(1,799)	(1,149)
Net Cash Flows Used in Investing Activities						(2,084,007)	(1,258,899)	(585,059)
Financing Activities:
Issuance of Common Shares, Net of Issuance Costs						928,992	852,254	0
Cash Dividends on Common Shares						(744,665)	(663,239)	(640,110)
Issuance of Long-Term Debt						1,550,000	0	1,550,000
Retirement of Long-Term Debt						0	(350,000)	(450,000)
Decrease in Notes Payable						(170,545)	593,370	(347,810)
Other Financing Activities						46,480	4,001	(12,455)
Net Cash Flows Provided by/(Used in) Financing Activities						1,610,262	436,386	99,625
Net (Decrease)/Increase in Cash						(1,035)	878	70
Cash, Cash Equivalents and Restricted Cash - Beginning of Year						1,469	591	521
Cash, Cash Equivalents and Restricted Cash - End of Year					$ 1,469	434	1,469	591
Cash Paid/(Received) During the Year for:
Interest						140,694	161,323	118,533
Income Taxes						$ (43,158)	$ (63,227)	$ (30,239)