SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - STATEMENT OF CASH FLOW (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities:
Net Income	$ 231,100	$ 267,200	$ 205,500	$ 246,000	$ 183,700	$ 237,800	$ 209,400	$ 255,100	$ 949,821	$ 886,004	$ 827,065
Flows Provided by Operating Activities:
Deferred Income Taxes									466,463	491,736	443,259
Other									(77,294)	(82,219)	56,026
Accounts Receivables from Subsidiaries									(142,699)	(39,797)	(122,139)
Taxes Receivable/Accrued, Net									234,543	30,282	(323,224)
Other Current Assets and Liabilities, Net									9,112	44,031	15,797
Net Cash Flows Provided by Operating Activities									2,175,052	1,433,751	1,651,976
Investing Activities:
Other Investing Activities									36,951	6,291	24,159
Net Cash Flows Used in Investing Activities									(2,150,808)	(1,659,150)	(1,591,795)
Financing Activities:
Cash Dividends on Common Shares									(564,486)	(529,791)	(475,227)
Issuance of Long-Term Debt									800,000	1,225,000	725,000
(Decrease)/Increase in Short-Term Debt									(12,453)	(242,122)	285,075
Other Financing Activities									(33,482)	(18,225)	(15,620)
Net Cash Flows (Used in)/Provided by Financing Activities									(17,940)	210,643	(64,842)
Net Increase/(Decrease) in Cash and Cash Equivalents									6,304	(14,756)	(4,661)
Cash and Cash Equivalents - Beginning of Year				23,947				38,703	23,947	38,703	43,364
Cash and Cash Equivalents - End of Year	30,251				23,947				30,251	23,947	38,703
Cash Paid/(Received) During the Year for:
Income Taxes									(135,500)	10,300	334,200
Eversource [Member]
Operating Activities:
Net Income									942,302	878,485	819,546
Flows Provided by Operating Activities:
Equity in Earnings of Subsidiaries									(922,321)	(900,824)	(848,435)
Cash Dividends Received from Subsidiaries									724,877	602,300	609,800
Deferred Income Taxes									19,008	16,880	7,956
Other									(27,963)	(22,864)	9,409
Accounts Receivables from Subsidiaries									(9,173)	(16,980)	88,800
Taxes Receivable/Accrued, Net									8,050	(14,426)	23,178
Accounts Payable, Including Affiliate Payables									(6,908)	(134,730)	5,942
Other Current Assets and Liabilities, Net									(7,433)	6,832	14,484
Net Cash Flows Provided by Operating Activities									720,439	414,673	730,680
Investing Activities:
Capital Contributions to Subsidiaries									(589,500)	(218,500)	(437,553)
Decrease/(Increase) in Notes Receivable from Subsidiaries									14,510	(131,650)	86,100
Other Investing Activities									0	12,000	0
Net Cash Flows Used in Investing Activities									(574,990)	(338,150)	(351,453)
Financing Activities:
Cash Dividends on Common Shares									(564,486)	(529,791)	(475,227)
Issuance of Long-Term Debt									500,000	450,000	0
(Decrease)/Increase in Short-Term Debt									(76,453)	(2,622)	86,575
Other Financing Activities									(4,484)	5,819	9,528
Net Cash Flows (Used in)/Provided by Financing Activities									(145,423)	(76,594)	(379,124)
Net Increase/(Decrease) in Cash and Cash Equivalents									26	(71)	103
Cash and Cash Equivalents - Beginning of Year				$ 67				$ 138	67	138	35
Cash and Cash Equivalents - End of Year	$ 93				$ 67				93	67	138
Cash Paid/(Received) During the Year for:
Interest									58,018	43,024	36,208
Income Taxes									$ (65,531)	$ (34,680)	$ (86,804)