SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENT OF CASH FLOW (Details) - USD ($) $ 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
Operating Activities:
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
Flows Provided by Operating Activities:
Deferred Income Taxes									174,812	491,630	466,463
Other									(111,225)	(204,421)	(118,195)
Accounts Receivables from Subsidiaries									(141,433)	(117,155)	(142,699)
Taxes Receivable/Accrued, Net									(67,770)	52,284	234,543
Other Current Assets and Liabilities, Net									78,226	91,545	13,737
Net Cash Flows Provided by Operating Activities									1,783,978	1,996,202	2,208,242
Investing Activities:
Other Investing Activities									6,754	5,479	36,211
Net Cash Flows Used in Investing Activities									(2,390,481)	(3,230,516)	(2,151,548)
Financing Activities:
Cash Dividends on Common Shares									(640,110)	(602,083)	(564,486)
Issuance of Long-Term Debt									2,200,000	2,500,000	800,000
Retirements of Long-Term Debt									(1,050,330)	(745,000)	(200,000)
Increase/(Decrease) in Short-Term Debt									(379,310)	72,810	(12,453)
Other Financing Activities									(28,457)	(4,754)	(33,482)
Net Cash Flows Provided by/(Used in) Financing Activities									729,937	1,213,454	(17,940)
Cash, Cash Equivalents and Restricted Cash - Beginning of Year				38,165					38,165
Cash, Cash Equivalents and Restricted Cash - End of Year	108,068				38,165				108,068	38,165
Cash Paid/(Received) During the Year for:
Interest									503,200	419,100	398,100
Income Taxes									158,800	30,800	(135,500)
Eversource
Operating Activities:
Net Income									1,033,000	987,996	942,302
Flows Provided by Operating Activities:
Equity in Earnings of Subsidiaries									(1,049,748)	(993,063)	(922,321)
Cash Dividends Received from Subsidiaries									569,500	753,300	724,877
Deferred Income Taxes									20,032	37,867	19,008
Other									(31,093)	(36,052)	(27,963)
Accounts Receivables from Subsidiaries									(28,716)	29,405	(9,173)
Taxes Receivable/Accrued, Net									(20,207)	1,555	8,050
Accounts Payable, Including Affiliate Payables									(9,817)	9,763	(6,908)
Other Current Assets and Liabilities, Net									2,553	7,536	(7,433)
Net Cash Flows Provided by Operating Activities									485,504	798,307	720,439
Investing Activities:
Capital Contributions to Subsidiaries									(955,700)	(1,156,731)	(589,500)
Return of Capital from Subsidiary									530,000	0	0
(Increase)/Decrease in Notes Receivable from Subsidiaries									(158,210)	(192,100)	14,510
Other Investing Activities									(1,149)	1,484	0
Net Cash Flows Used in Investing Activities									(585,059)	(1,347,347)	(574,990)
Financing Activities:
Cash Dividends on Common Shares									(640,110)	(602,083)	(564,486)
Issuance of Long-Term Debt									1,550,000	1,200,000	500,000
Retirements of Long-Term Debt									(450,000)	0	0
Increase/(Decrease) in Short-Term Debt									(347,810)	(42,690)	(76,453)
Other Financing Activities									(12,455)	(5,759)	(4,484)
Net Cash Flows Provided by/(Used in) Financing Activities									99,625	549,468	(145,423)
Net Increase/(Decrease) in Cash and Cash Equivalents									70	428	26
Cash, Cash Equivalents and Restricted Cash - Beginning of Year				$ 521				$ 93	521	93	67
Cash, Cash Equivalents and Restricted Cash - End of Year	$ 591				$ 521				591	521	93
Cash Paid/(Received) During the Year for:
Interest									118,533	73,868	58,018
Income Taxes									$ (30,239)	$ (59,526)	$ (65,531)