SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENT OF CASH FLOW (Details) - USD ($) $ in Thousands			3 Months Ended								12 Months Ended
	Dec. 30, 2019	Jun. 04, 2019	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
Operating Activities:
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
Flows Provided by Operating Activities:
Equity in Earnings of Subsidiaries											(42,200)	(3,800)	(27,400)
Deferred Income Taxes											209,812	174,812	491,630
Other											(196,087)	(111,225)	(204,421)
Accounts Receivables from Subsidiaries											(98,716)	(141,433)	(117,155)
Taxes Receivable/Accrued, Net											(16,129)	(67,770)	52,284
Other Current Assets and Liabilities, Net											(11,603)	78,226	91,545
Net Cash Flows Provided by Operating Activities											2,009,577	1,830,543	1,996,202
Investing Activities:
Other Investing Activities											24,204	6,754	5,479
Net Cash Flows Used in Investing Activities											(3,274,288)	(2,437,046)	(3,230,516)
Financing Activities:
Issuance of Common Shares, Net of Issuance Costs	$ 425,400	$ 426,900									852,254	0	0
Cash Dividends on Common Shares											(663,239)	(640,110)	(602,083)
Issuance of Long-Term Debt											1,520,000	2,200,000	2,500,000
Retirement of Long-Term Debt											(801,078)	(1,050,330)	(745,000)
Increase/(Decrease) in Notes Payable											325,370	(379,310)	72,810
Other Financing Activities											(1,006)	(28,457)	(4,754)
Net Cash Flows Provided by/(Used in) Financing Activities											1,172,450	729,937	1,213,454
Cash, Cash Equivalents and Restricted Cash - Beginning of Year						108,068					108,068
Cash, Cash Equivalents and Restricted Cash - End of Year			15,432				108,068				15,432	108,068
Cash Paid/(Received) During the Year for:
Interest											532,400	503,200	419,100
Income Taxes											56,000	158,800	30,800
Eversource
Operating Activities:
Net Income											909,053	1,033,000	987,996
Flows Provided by Operating Activities:
Equity in Earnings of Subsidiaries											(1,001,526)	(1,049,748)	(993,063)
Cash Dividends Received from Subsidiaries											883,000	569,500	753,300
Deferred Income Taxes											13,382	20,032	37,867
Other											19,584	(31,093)	(36,052)
Accounts Receivables from Subsidiaries											7,105	(28,716)	29,405
Taxes Receivable/Accrued, Net											(605)	(20,207)	1,555
Accounts Payable to Subsidiaries											(4,099)	(9,817)	9,763
Other Current Assets and Liabilities, Net											(2,503)	2,553	7,536
Net Cash Flows Provided by Operating Activities											823,391	485,504	798,307
Investing Activities:
Capital Contributions to Subsidiaries											(1,039,000)	(955,700)	(1,156,731)
Return of Capital from Subsidiary											0	530,000	0
Increase in Notes Receivable from Subsidiaries											(218,100)	(158,210)	(192,100)
Other Investing Activities											(1,799)	(1,149)	1,484
Net Cash Flows Used in Investing Activities											(1,258,899)	(585,059)	(1,347,347)
Financing Activities:
Issuance of Common Shares, Net of Issuance Costs											852,254	0	0
Cash Dividends on Common Shares											(663,239)	(640,110)	(602,083)
Issuance of Long-Term Debt											0	1,550,000	1,200,000
Retirement of Long-Term Debt											(350,000)	(450,000)	0
Increase/(Decrease) in Notes Payable											593,370	(347,810)	(42,690)
Other Financing Activities											4,001	(12,455)	(5,759)
Net Cash Flows Provided by/(Used in) Financing Activities											436,386	99,625	549,468
Net Increase/(Decrease) in Cash and Cash Equivalents											878	70	428
Cash, Cash Equivalents and Restricted Cash - Beginning of Year						$ 591				$ 521	591	521	93
Cash, Cash Equivalents and Restricted Cash - End of Year			$ 1,469				$ 591				1,469	591	521
Cash Paid/(Received) During the Year for:
Interest											161,323	118,533	73,868
Income Taxes											$ (63,227)	$ (30,239)	$ (59,526)