SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions					12 Months Ended
	Dec. 04, 2017 USD ($)	Oct. 11, 2017 agreement	Jun. 02, 2017 USD ($)	Mar. 25, 2016 USD ($)	Dec. 31, 2017 USD ($) customer utility company MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric and natural gas customers | customer					4
Payments to acquire businesses					$ 877,652	$ 0	$ 0
Goodwill					$ 4,427,266	3,519,401
Capacity required for transmission line | MW					1,090
Amount capitalized in transmission line installation					$ 277,000
Equity method investment					$ 277,600	236,900
Number of companies that transmit electricity imported from hydro-quebec system | company					2
Number of nuclear generation companies | company					3
Retained Earnings					$ 3,561,084	3,175,171
Net of damages awarded				$ 26,100
Eversource
Summary of Accounting Policies [Line Items]
Goodwill					3,231,811	3,231,811
Equity method investment					10,945,986	9,703,287
Retained Earnings					3,561,084	3,175,171
The Connecticut Light And Power Company
Summary of Accounting Policies [Line Items]
Cash collateral deposited					3,100	1,400
Retained Earnings					1,415,741	1,299,374
Net of damages awarded				13,600
Damages paid							244,600
Accumulated interest from damages awarded							178,000
Long term receivable					25,000
NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Cash collateral deposited					12,800	11,800
Retained Earnings					1,944,961	1,844,195
Net of damages awarded				8,600
Damages paid							57,400
Accumulated interest from damages awarded							41,800
Long term receivable					3,800
Public Service Company Of New Hampshire
Summary of Accounting Policies [Line Items]
Cash collateral deposited					500	500
Retained Earnings					511,382	549,286
Net of damages awarded				$ 3,900
Long term receivable					5,500
Eversource
Summary of Accounting Policies [Line Items]
Cash collateral deposited					24,500	21,700
Retained Earnings					27,400	200	$ 900
Dividends					$ 20,000	100
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility					8
FERC Regulated Transmission Business
Summary of Accounting Policies [Line Items]
Equity method investment					$ 159,600	$ 154,600
Equity method investment, ownership percentage					15.00%	15.00%
Algonquin Gas Tansmsission LLC - Access Northeast
Summary of Accounting Policies [Line Items]
Equity method investment					$ 31,300	$ 30,900
Equity method investment, ownership percentage					40.00%	40.00%
Hydro-Quebec System
Summary of Accounting Policies [Line Items]
Equity method investment					$ 17,700	$ 7,700
Equity method investment, ownership percentage					37.20%	37.20%
Hydro-Quebec System | NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Equity method investment					$ 6,900	$ 3,000
Equity method investment, ownership percentage					14.50%	14.50%
Yankee Companies
Summary of Accounting Policies [Line Items]
Net of damages awarded					$ 52,200
CYAPC and YAEC
Summary of Accounting Policies [Line Items]
Total amount of damages awarded					6,800
MYAPC
Summary of Accounting Policies [Line Items]
Net of damages awarded					14,400
Other Equity Method Investments
Summary of Accounting Policies [Line Items]
Equity method investment					69,000	$ 43,700
Aquarion Water Company
Summary of Accounting Policies [Line Items]
Consideration transferred			$ 1,675,000
Payments to acquire businesses	$ 880,000		880,000
Consideration transferred, liabilities incurred			$ 795,000
Goodwill	$ 900,000				907,900
Subsequent Event
Summary of Accounting Policies [Line Items]
Number of purchase sale agreements | agreement		2
Available-for-sale Securities | Accounting Standards Update 2016-01 | Retained Earnings
Summary of Accounting Policies [Line Items]
Assets subject to accounting					51,000
Unrealized Holding Loss on Investments | Accounting Standards Update 2016-01 | Retained Earnings
Summary of Accounting Policies [Line Items]
Cumulative effect of new accounting principle in period of adoption					$ 100