SHORT-TERM AND LONG-TERM DEBT - Summary of Long-Term Debt Issuance and Repayments (Details) - USD ($) $ in Thousands		1 Months Ended								9 Months Ended
	Aug. 10, 2021	Oct. 31, 2021	Sep. 30, 2021	Aug. 31, 2021	Jun. 30, 2021	May 31, 2021	Apr. 30, 2021	Mar. 31, 2021	Feb. 28, 2021	Sep. 30, 2021	Sep. 30, 2020
Debt Instrument [Line Items]
Repayments										$ (1,142,500)	$ (302,236)
Senior Notes | Eversource Parent 2.50% Series I Senior Notes | Eversource Parent
Debt Instrument [Line Items]
Interest rate, stated percentage									2.50%
Repayments									$ (450,000)
Senior Notes | Eversource Parent 2.55% Series S Senior Notes | Eversource Parent
Debt Instrument [Line Items]
Interest rate, stated percentage								2.55%
Issuances								$ 350,000
Senior Notes | Eversource Parent 1.40% Series U Senior Notes | Eversource Parent
Debt Instrument [Line Items]
Interest rate, stated percentage				1.40%
Issuances				$ 300,000
Senior Notes | Eversource Parent Variable Rate Series T Senior Notes | Eversource Parent
Debt Instrument [Line Items]
Issuances	$ 350,000			$ 350,000
Senior Notes | Eversource Parent Variable Rate Series T Senior Notes | Eversource Parent | Compounded SOFR
Debt Instrument [Line Items]
Basis spread on variable rate	0.25%
CL&P
Debt Instrument [Line Items]
Repayments										$ (120,500)	0
CL&P | Senior Notes | 2.05% Series A First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate, stated percentage					2.05%
Issuances					$ 425,000
CL&P | Senior Notes | 4.38% Series A PCRB
Debt Instrument [Line Items]
Interest rate, stated percentage			4.38%							4.38%
Repayments			$ (120,500)
NSTAR Electric
Debt Instrument [Line Items]
Repayments										$ (250,000)	(95,000)
NSTAR Electric | Senior Notes | 3.10% 2021 Debentures
Debt Instrument [Line Items]
Interest rate, stated percentage						3.10%
Issuances						$ 300,000
NSTAR Electric | Senior Notes | 3.50% Series F Senior Notes
Debt Instrument [Line Items]
Interest rate, stated percentage					3.50%
Repayments					$ (250,000)
NSTAR Electric | Senior Notes | 1.95% 2021 Debentures
Debt Instrument [Line Items]
Interest rate, stated percentage				1.95%
Issuances				$ 300,000
PSNH
Debt Instrument [Line Items]
Repayments										(282,000)	$ 0
PSNH | Senior Notes | 4.05% Series Q First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate, stated percentage								4.05%
Repayments								$ (122,000)
PSNH | Senior Notes | 3.20% Series R First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate, stated percentage					3.20%
Repayments					$ (160,000)
PSNH | Senior Notes | 2.20% Series V First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate, stated percentage					2.20%
Issuances					$ 350,000
Aquarion | Senior Notes | Aquarion Water Company of Connecticut 3.31% Senior Notes
Debt Instrument [Line Items]
Interest rate, stated percentage							3.31%
Issuances							$ 100,000
Aquarion | Senior Notes | Aquarion Water Company of Connecticut 5.50% Notes
Debt Instrument [Line Items]
Interest rate, stated percentage							5.50%
Repayments							$ (40,000)
EGMA | Senior Notes
Debt Instrument [Line Items]
Debt instrument, refinanced amount			$ 309,400							$ 309,400
EGMA | Senior Notes | EGMA 2.11% Series A First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate, stated percentage			2.11%							2.11%
Issuances			$ 310,000
EGMA | Senior Notes | EGMA 2.92% Series B First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate, stated percentage			2.92%							2.92%
Issuances			$ 240,000
Yankee Gas | Senior Notes | Yankee Gas 1.38% Series S First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate, stated percentage				1.38%
Issuances				$ 90,000
Yankee Gas | Senior Notes | Yankee Gas 2.88% Series T First Mortgage Bonds
Debt Instrument [Line Items]
Interest rate, stated percentage				2.88%
Issuances				$ 35,000
NSTAR Gas Company | Senior Notes | NSTAR Gas 2.25% Series T First Mortgage Bonds | Subsequent Event
Debt Instrument [Line Items]
Interest rate, stated percentage		2.25%
Issuances		$ 40,000
NSTAR Gas Company | Senior Notes | NSTAR Gas 3.03% Series U First Mortgage Bonds | Subsequent Event
Debt Instrument [Line Items]
Interest rate, stated percentage		3.03%
Issuances		$ 40,000