SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENT OF CASH FLOW (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities:
Net Income	$ 239,400	$ 262,200	$ 232,600	$ 261,300	$ 231,100	$ 267,200	$ 205,500	$ 246,000	$ 995,515	$ 949,821	$ 886,004
Flows Provided by Operating Activities:
Deferred Income Taxes									491,630	466,463	491,736
Other									(148,429)	(77,294)	(82,219)
Accounts Receivables from Subsidiaries									(117,155)	(142,699)	(39,797)
Taxes Receivable/Accrued, Net									52,284	234,543	30,282
Other Current Assets and Liabilities, Net									88,738	9,112	44,031
Net Cash Flows Provided by Operating Activities									2,004,934	2,175,052	1,433,751
Investing Activities:
Other Investing Activities									25,521	36,951	6,291
Net Cash Flows Used in Investing Activities									(3,210,474)	(2,150,808)	(1,659,150)
Financing Activities:
Cash Dividends on Common Shares									(602,083)	(564,486)	(529,791)
Issuance of Long-Term Debt									2,500,000	800,000	1,225,000
Decrease in Short-Term Debt									72,810	(12,453)	(242,122)
Other Financing Activities									(4,754)	(33,482)	(18,225)
Net Cash Flows Provided by/(Used in) Financing Activities									1,213,454	(17,940)	210,643
Net Increase/(Decrease) in Cash and Cash Equivalents									7,914	6,304	(14,756)
Cash and Cash Equivalents - Beginning of Year				30,251				23,947	30,251	23,947	38,703
Cash and Cash Equivalents - End of Year	38,165				30,251				38,165	30,251	23,947
Cash Paid/(Received) During the Year for:
Income Taxes									30,800	(135,500)	10,300
Eversource
Operating Activities:
Net Income									987,996	942,302	878,485
Flows Provided by Operating Activities:
Equity in Earnings of Subsidiaries									(993,063)	(922,321)	(900,824)
Cash Dividends Received from Subsidiaries									753,300	724,877	602,300
Deferred Income Taxes									37,867	19,008	16,880
Other									(36,052)	(27,963)	(22,864)
Accounts Receivables from Subsidiaries									29,405	(9,173)	(16,980)
Taxes Receivable/Accrued, Net									1,555	8,050	(14,426)
Accounts Payable, Including Affiliate Payables									9,763	(6,908)	(134,730)
Other Current Assets and Liabilities, Net									7,536	(7,433)	6,832
Net Cash Flows Provided by Operating Activities									798,307	720,439	414,673
Investing Activities:
Capital Contributions to Subsidiaries									(1,156,731)	(589,500)	(218,500)
(Increase)/Decrease in Notes Receivable from Subsidiaries									(192,100)	14,510	(131,650)
Other Investing Activities									1,484	0	12,000
Net Cash Flows Used in Investing Activities									(1,347,347)	(574,990)	(338,150)
Financing Activities:
Cash Dividends on Common Shares									(602,083)	(564,486)	(529,791)
Issuance of Long-Term Debt									1,200,000	500,000	450,000
Decrease in Short-Term Debt									(42,690)	(76,453)	(2,622)
Other Financing Activities									(5,759)	(4,484)	5,819
Net Cash Flows Provided by/(Used in) Financing Activities									549,468	(145,423)	(76,594)
Net Increase/(Decrease) in Cash and Cash Equivalents									428	26	(71)
Cash and Cash Equivalents - Beginning of Year				$ 93				$ 67	93	67	138
Cash and Cash Equivalents - End of Year	$ 521				$ 93				521	93	67
Cash Paid/(Received) During the Year for:
Interest									73,868	58,018	43,024
Income Taxes									$ (59,526)	$ (65,531)	$ (34,680)