LONG-TERM DEBT - Schedule of Issuances and Repayments (Details) - USD ($) $ in Thousands	1 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Aug. 31, 2020	Jul. 30, 2020	May 31, 2020	Apr. 30, 2020	Mar. 31, 2020	Jan. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Repayments of debt									$ (327,236)	$ (801,078)	$ (1,050,330)
Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 15,125,900								15,125,900	13,770,800
The Connecticut Light and Power Company
Debt Instrument [Line Items]
Repayments of debt									0	(250,000)	(300,000)
The Connecticut Light and Power Company | Senior Notes | 0.75% Series A First Mortgage Bonds
Debt Instrument [Line Items]
Issuance	$ 400,000
NSTAR Electric Company
Debt Instrument [Line Items]
Repayments of debt									(95,000)	0	0
NSTAR Electric Company | Senior Notes | 3.95% 2020 Debentures
Debt Instrument [Line Items]
Interest rate							3.95%
Issuance							$ 400,000
NSTAR Electric Company | Senior Notes | 5.10% Series E Senior Notes
Debt Instrument [Line Items]
Interest rate							5.10%
Repayments of debt							$ (95,000)
Public Service Company of New Hampshire
Debt Instrument [Line Items]
Repayments of debt									$ 0	(150,000)	(199,250)
Public Service Company of New Hampshire | Senior Notes | 2.40% Series U First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate			2.40%
Issuance			$ 150,000
NSTAR Gas | Senior Notes | Eversource Parent Series I Senior Notes
Debt Instrument [Line Items]
Interest rate								2.50%
Repayments of debt								$ (450,000)
NSTAR Gas | Senior Notes | NSTAR Gas 4.46% Series N First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate								4.46%
Repayments of debt								$ (125,000)
NSTAR Gas | Senior Notes | NSTAR Gas 2.33% Series R First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate					2.33%
Issuance					$ 75,000
NSTAR Gas | Senior Notes | NSTAR Gas 3.15% Series S First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate					3.15%
Issuance					$ 115,000
NSTAR Gas | Senior Notes | NSTAR Gas 9.95% Series J First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate	9.95%								9.95%
Repayments of debt	$ (25,000)
Yankee Gas | Senior Notes | Yankee Gas 4.87% Series K First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate						4.87%
Repayments of debt						$ (50,000)
Yankee Gas | Senior Notes | Yankee Gas 2.90% Series R First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate		2.90%
Issuance		$ 70,000
Aquarion | Senior Notes | Aquarion Water Company of Massachusetts, Inc. and Aquarion Water Capital of Massachusetts, Inc. various term loans and general mortgage bonds
Debt Instrument [Line Items]
Repayments of debt				$ (32,200)
Eversource
Debt Instrument [Line Items]
Repayments of debt									$ 0	$ (350,000)	$ (450,000)
Eversource | Senior Notes | Eversource Parent 3.45% Series P Senior Notes
Debt Instrument [Line Items]
Interest rate			3.45%					3.45%
Issuance			$ 300,000					$ 350,000
Eversource | Senior Notes | Eversource Parent 0.80% Series Q Senior Notes
Debt Instrument [Line Items]
Interest rate			0.80%
Issuance			$ 300,000
Eversource | Senior Notes | Eversource Parent 1.65% Series R Senior Notes
Debt Instrument [Line Items]
Interest rate			1.65%
Issuance			$ 600,000
Eversource | The Connecticut Light and Power Company | Senior Notes | 4.00% 2018 Series A First Mortgage Bonds
Debt Instrument [Line Items]
Long-term debt	$ 650,000								$ 650,000