REGULATORY ACCOUNTING - Narrative (Details)							4 Months Ended	12 Months Ended			37 Months Ended
	Dec. 26, 2023 USD ($)	Oct. 30, 2023 USD ($)	May 01, 2023 USD ($)	Jan. 01, 2023 USD ($)	Nov. 30, 2022 USD ($)	Oct. 31, 2022 USD ($)	Apr. 30, 2023 USD ($)	Dec. 31, 2023 USD ($) storm	Dec. 31, 2022 USD ($) storm	Dec. 31, 2021 USD ($)	Dec. 31, 2021 weather_event	Dec. 22, 2023 USD ($)	Mar. 31, 2023 USD ($)	May 31, 2018 USD ($)
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								$ 241,700,000	$ 210,800,000
Deferred costs								82,100,000	64,000,000
Gain (loss) related to litigation settlement								0	72,041,000	$ (81,274,000)
Regulatory assets								6,389,200,000	5,578,300,000
Increase (decrease) in regulatory assets								151,548,000	205,294,000	314,211,000
FERC ROE Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers								39,100,000	39,100,000
Unfavorable Regulatory Action
Regulatory Assets [Line Items]
Gain (loss) related to litigation settlement										(103,600,000)
Unfavorable Regulatory Action | Oil and Gas, Operation and Maintenance
Regulatory Assets [Line Items]
Gain (loss) related to litigation settlement										(10,000,000)
Deferred Pre-Staging And Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								542,000,000
Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								$ 1,750,000,000
Goodwill-related
Regulatory Assets [Line Items]
Recovery period (in years)								40 years
Remaining amortization (in years)								16 years
Regulatory assets								$ 264,100,000	281,000,000.0
PSNH
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								1,200,000	1,000,000
Equity return not recorded on the balance sheet								10,200,000	4,100,000
Regulatory assets								963,300,000	696,200,000
Increase (decrease) in regulatory assets								273,472,000	(53,181,000)	(32,587,000)
PSNH | FERC ROE Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers								3,100,000	3,100,000
PSNH | Deferred Pre-Staging And Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								172,000,000
PSNH | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								246,000,000
PSNH | Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets								0	0
PSNH | Operation, Maintenance And Vegetation Expenses
Regulatory Assets [Line Items]
Regulatory assets			$ 16,900,000
Purchase price of regulatory assets			$ 23,300,000
CL&P
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								166,700,000	135,900,000
Deferred costs								64,000,000	52,800,000
Average monthly bill							$ 10
Public utilities, approved rate increase (decrease), net revenues generated							$ 90,000,000
Gain (loss) related to litigation settlement								0	72,041,000	(81,274,000)
Regulatory assets								2,143,200,000	1,907,800,000
Increase (decrease) in regulatory assets								(157,200,000)	144,793,000	152,775,000
CL&P | Energy Assistance Program
Regulatory Assets [Line Items]
Restricted cash													$ 10,000,000
CL&P | FERC ROE Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers								21,400,000	21,400,000
CL&P | Unfavorable Regulatory Action | Oil and Gas, Operation and Maintenance
Regulatory Assets [Line Items]
Gain (loss) related to litigation settlement									(10,000,000)
CL&P | Deferred Pre-Staging And Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								178,000,000
CL&P | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								975,000,000
CL&P | Deferred Storm Restoration Costs | Tropical Storm Isaias
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								232,000,000
CL&P | Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets								0	0
CL&P | Catastrophic Storm Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved												$ 634,000,000
Number of catastrophic weather events | weather_event											24
Annual amount of regulatory costs collected												$ 50,000,000
NSTAR Electric
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								21,900,000	19,800,000
Deferred costs								$ 7,300,000	$ 3,500,000
Rate increase approved	$ 104,900,000				$ 64,000,000
Renewal term of performance-based ratemaking plan (in years)									5 years
Extension period (in years)									5 years
Historical average term of capital additions (in years)								5 years	5 years
Authorized regulatory ROE (in percent)									9.80%
Authorized common equity ratio (in percent)									53.20%
Regulatory assets								$ 1,957,900,000	$ 1,714,400,000
Increase (decrease) in regulatory assets								$ 141,865,000	$ 88,220,000	$ 79,075,000
Storms in a calendar year | storm								8
Storms recovered at base rate | storm									6
Number of additional storms | storm									2
NSTAR Electric | FERC ROE Complaints
Regulatory Assets [Line Items]
Cumulative pre-tax reserves which include impact of refunds given to customers								$ 14,600,000	$ 14,600,000
NSTAR Electric | Deferred Pre-Staging And Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								192,000,000
NSTAR Electric | Deferred Storm Restoration Costs
Regulatory Assets [Line Items]
Amount of regulatory costs not yet approved								526,000,000
NSTAR Electric | Goodwill-related
Regulatory Assets [Line Items]
Regulatory assets								$ 226,700,000	241,200,000
NSTAR Electric | Storm Fund Contribution
Regulatory Assets [Line Items]
Regulatory assets									10,000,000
Increase (decrease) in regulatory assets									31,000,000
NSTAR Electric | Storm Threshold Costs
Regulatory Assets [Line Items]
Regulatory assets									1,300,000
NSTAR Electric | Recovery of Storm Costs
Regulatory Assets [Line Items]
Regulatory assets				$ 162,100,000
Recovery period of regulatory asset (in years)				5 years
NSTAR Electric | Recovery of exogenous storms
Regulatory Assets [Line Items]
Regulatory assets									$ 220,900,000
Recovery period of regulatory asset (in years)									5 years
NSTAR Electric | Recovery of historical exogenous property taxes from 2020 - 2022
Regulatory Assets [Line Items]
Regulatory assets				$ 30,800,000
Recovery period of regulatory asset (in years)				2 years
NSTAR Electric | Recovery of historical exogenous property taxes from 2012 - 2015
Regulatory Assets [Line Items]
Regulatory assets				$ 8,300,000
Recovery period of regulatory asset (in years)				5 years
NSTAR Electric | Inflation Based Adjustment
Regulatory Assets [Line Items]
Rate increase approved	50,600,000
NSTAR Electric | K-Bar Adjustment
Regulatory Assets [Line Items]
Rate increase approved	$ 54,300,000
NSTAR Gas
Regulatory Assets [Line Items]
Rate increase approved		$ 25,400,000				$ 21,700,000
NSTAR Gas | Exogenous Property Taxes
Regulatory Assets [Line Items]
Rate increase approved						4,500,000
NSTAR Gas | Mitigation Plan
Regulatory Assets [Line Items]
Rate increase approved						5,400,000
NSTAR Gas | Recovery of Exogenous Property Taxes
Regulatory Assets [Line Items]
Regulatory assets						$ 8,200,000
Recovery period of regulatory asset (in years)						2 years
NSTAR Gas | Base Rate Adjustment
Regulatory Assets [Line Items]
Rate increase approved		$ 15,500,000
EGMA
Regulatory Assets [Line Items]
Rate increase approved						$ 6,700,000
EGMA | Tax Act Credit Factor
Regulatory Assets [Line Items]
Rate increase approved						3,300,000
EGMA | Recovery of Exogenous Property Taxes
Regulatory Assets [Line Items]
Regulatory assets						$ 8,600,000
Recovery period of regulatory asset (in years)						2 years
Rate Reduction Bonds | PSNH
Regulatory Assets [Line Items]
Amount of securitized rate reduction bonds issued														$ 635,700,000