SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands		1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 25, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) customer	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) company utility MW	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Feb. 08, 2019 USD ($)	Jan. 01, 2019 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric and natural gas customers | customer			4,000,000
Goodwill		$ 4,427,266	$ 4,427,266		$ 4,427,266	$ 4,427,266
Other Income, Net					128,366	107,913	$ 64,505
Equity method investment		463,700	463,700		$ 463,700	277,600
Number of Companies That Transmit Electricity Imported From Hydro-Quebec System | company					2
Other-than-temporary impairment				$ 32,900	$ 32,900
Number of nuclear generation companies | company					3
Net of damages awarded	$ 26,100
The Connecticut Light And Power Company
Summary of Accounting Policies [Line Items]
Other Income, Net					$ 22,663	22,991	14,238
Period of accounts receivable recoverable under financial or medical duress					180 days
Equity method investment		1,300	1,300		$ 1,300
Net of damages awarded	13,600
Long term receivable		25,000	25,000		25,000	25,000
Transfer of loans receivable		41,300
Maximum loans receivable outstanding		55,000	55,000		55,000
Loans transferred receivable current		18,500	18,500		18,500
Loans transferred receivable non-current		22,800	22,800		$ 22,800
Yankee Gas Services Company
Summary of Accounting Policies [Line Items]
Period of accounts receivable recoverable under financial or medical duress					90 days
NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Other Income, Net					$ 53,066	34,100	21,263
Equity method investment		900	900		900
Net of damages awarded	8,600
Long term receivable		5,500	5,500		5,500	5,500
Public Service Company Of New Hampshire
Summary of Accounting Policies [Line Items]
Other Income, Net					27,672	9,805	7,563
Equity method investment		300	300		300
Net of damages awarded	$ 3,900
Long term receivable		3,800	3,800		3,800	3,800
Eversource
Summary of Accounting Policies [Line Items]
Dividends					$ 22,300	20,000	100
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility					8
Northern Pass
Summary of Accounting Policies [Line Items]
Capacity required for transmission line | MW					1,090
Capitalized Costs, Proved Properties		$ 307,000	$ 307,000		$ 307,000
Bay State Wind LLC
Summary of Accounting Policies [Line Items]
Ownership interest		50.00%	50.00%		50.00%
Equity method investment		$ 234,300	$ 234,300		$ 234,300	$ 30,200
FERC Regulated Transmission Business
Summary of Accounting Policies [Line Items]
Ownership interest						15.00%
Equity method investment		$ 155,000	$ 155,000		$ 155,000	$ 159,600
Algonquin Gas Tansmsission LLC - Access Northeast
Summary of Accounting Policies [Line Items]
Ownership interest		40.00%	40.00%		40.00%
Equity method investment						31,300
Hydro-Quebec System
Summary of Accounting Policies [Line Items]
Ownership interest		37.20%	37.20%		37.20%
Equity method investment		$ 19,500	$ 19,500		$ 19,500	17,700
Hydro-Quebec System | NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Ownership interest		14.50%	14.50%		14.50%
Equity method investment		$ 7,600	$ 7,600		$ 7,600	6,900
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		54,900	54,900		54,900	38,800
Accounting Standards Update 2014-09
Summary of Accounting Policies [Line Items]
Operating Revenue and Purchased Power, Fuel and Transmission		$ 22,800	$ 22,800		22,800
Accounting Standards Update 2016-01
Summary of Accounting Policies [Line Items]
Unrealized gains recorded in other income					4,300
Accounting Standards Update 2017-07
Summary of Accounting Policies [Line Items]
Increase (Decrease) in Regulatory Liabilities					39,800
Accounting Standards Update 2016-15
Summary of Accounting Policies [Line Items]
Proceeds from Equity Method Investment, Distribution					19,100	20,000	700
Equity in Earnings
Summary of Accounting Policies [Line Items]
Other Income, Net					3,800	27,400	200
Equity in Earnings | The Connecticut Light And Power Company
Summary of Accounting Policies [Line Items]
Other Income, Net					100	0	100
Equity in Earnings | NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Other Income, Net					700	300	300
Equity in Earnings | Public Service Company Of New Hampshire
Summary of Accounting Policies [Line Items]
Other Income, Net					$ 0	$ 0	$ 0
Subsequent Event | Revolution Wind And South Fork Wind Power
Summary of Accounting Policies [Line Items]
Ownership interest								50.00%
Parent obligations								$ 127,600
Scenario, Forecast | Accounting Standards Update 2016-19
Summary of Accounting Policies [Line Items]
Right of use asset									$ 60,000
Scenario, Forecast | Accounting Standards Update 2016-19 | The Connecticut Light And Power Company
Summary of Accounting Policies [Line Items]
Right of use asset									1,000
Scenario, Forecast | Accounting Standards Update 2016-19 | NSTAR Electric Company
Summary of Accounting Policies [Line Items]
Right of use asset									25,000
Scenario, Forecast | Accounting Standards Update 2016-19 | Public Service Company Of New Hampshire
Summary of Accounting Policies [Line Items]
Right of use asset									$ 1,000