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REGULATORY ACCOUNTING - Narrative (Details) |  |  |  |  |  |  | 4 Months Ended | 12 Months Ended | 37 Months Ended |  |  |
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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

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