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REGULATORY ACCOUNTING - Narrative (Details) $ in Millions |  |  |  |  |  |  |  |  |  |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 01, 2025 USD ($) | Dec. 23, 2024 USD ($) | Nov. 07, 2024 USD ($) | Oct. 30, 2024 USD ($) | Aug. 01, 2024 USD ($) | Jun. 11, 2024 USD ($) | Dec. 31, 2023 USD ($) | Dec. 26, 2023 USD ($) | Oct. 30, 2023 USD ($) | May 01, 2023 USD ($) | Dec. 31, 2024 USD ($) storm | Dec. 31, 2023 USD ($) | Aug. 14, 2024 USD ($) | Mar. 28, 2024 USD ($) | May 31, 2018 USD ($)
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  | $ 241.7 |  |  |  | $ 221.0 | $ 241.7 |  |  |
Deferred costs |  |  |  |  |  |  | 82.1 |  |  |  | 92.5 | 82.1 |  |  |
Regulatory assets |  |  |  |  |  |  | 6,389.2 |  |  |  | 7,070.7 | 6,389.2 |  |  |
FERC ROE Complaints |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers |  |  |  |  |  |  |  |  |  |  | 39.1 | 39.1 |  |  |
Deferred Pre-Staging And Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | 410.0 |  |  |  |
Deferred Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | $ 2,100.0 |  |  |  |
Goodwill-related |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery period (in years) |  |  |  |  |  |  |  |  |  |  | 40 years |  |  |  |
Remaining amortization (in years) |  |  |  |  |  |  |  |  |  |  | 15 years |  |  |  |
Regulatory assets |  |  |  |  |  |  | 264.1 |  |  |  | $ 247.2 | 264.1 |  |  |
PSNH |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  | 1.2 |  |  |  | 4.5 | 1.2 |  |  |
Equity return not recorded on the balance sheet |  |  |  |  |  |  | 10.2 |  |  |  | $ 22.5 | 10.2 |  |  |
Public utilities, request rate, unrecovered costs |  |  |  |  |  | $ 247.0 |  |  |  |  |  |  |  |  |
Public utilities, requested rate, unrecovered costs, recovery period |  |  |  |  |  | 5 years |  |  |  |  |  |  |  |  |
Public utilities, annual rate adjustment term |  |  |  |  |  |  |  |  |  |  | 4 years |  |  |  |
Public utilities, commitment agreement term |  |  |  |  |  | 4 years |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  | 963.3 |  |  |  | $ 1,065.7 | 963.3 |  |  |
PSNH | FERC ROE Complaints |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers |  |  |  |  |  |  |  |  |  |  | 3.1 | 3.1 |  |  |
PSNH | Deferred Pre-Staging And Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | 191.0 |  |  |  |
PSNH | Deferred Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | 440.0 |  |  |  |
PSNH | Goodwill-related |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  | 0.0 |  |  |  | 0.0 | 0.0 |  |  |
PSNH | Base Rate Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase (decrease) requested |  |  |  |  |  | $ 181.9 |  |  |  |  |  |  |  |  |
Rate increase approved |  |  |  |  | $ 61.2 |  |  |  |  |  |  |  |  |  |
PSNH | Operation, Maintenance And Vegetation Expenses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase price of regulatory assets |  |  |  |  |  |  |  |  |  | $ 23.3 |  |  |  |  |
Regulatory assets |  |  |  |  |  |  |  |  |  | $ 16.9 |  |  |  |  |
CL&P |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  | 166.7 |  |  |  | 116.3 | 166.7 |  |  |
Deferred costs |  |  |  |  |  |  | 64.0 |  |  |  | 47.2 | 64.0 |  |  |
Public utilities, approved rate, recovery of incurred and deferred program costs |  |  |  |  |  |  |  |  |  |  |  |  | $ 44.4 |  |
Public utilities, approved rate, expected program costs |  |  |  |  |  |  |  |  |  |  |  |  | 24.3 |  |
Public utilities, approved rate, recovery of incurred and deferred program costs, carrying costs |  |  |  |  |  |  |  |  |  |  |  |  | $ 5.4 |  |
Public utilities, approved rate, recovery of incurred and deferred program costs, period |  |  |  |  |  |  |  |  |  |  |  |  | 20 months |  |
Public utilities, approved rate, expected program costs, period |  |  |  |  |  |  |  |  |  |  |  |  | 8 months |  |
Regulatory assets |  |  |  |  |  |  | 2,143.2 |  |  |  | 2,325.5 | 2,143.2 |  |  |
CL&P | SBC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate, recovery, net of under recoveries |  |  |  |  |  |  | 86.2 |  |  |  |  | 86.2 |  |  |
Public utilities, approved rate, recovery of expected net costs |  |  |  |  |  |  | 254.4 |  |  |  |  | 254.4 |  |  |
CL&P | NBFMCC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate, recovery, net of under recoveries |  |  |  |  |  |  | 264.9 |  |  |  |  | 264.9 |  |  |
Public utilities, approved rate, recovery of expected net costs |  |  |  |  |  |  | 388.5 |  |  |  |  | 388.5 |  |  |
CL&P | 2024 Innovative Energy Solutions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate, incremental program costs |  |  |  |  |  |  |  |  |  |  |  |  | $ 3.5 |  |
CL&P | Connecticut Green Bank |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, approved rate, incremental program costs |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.5 |  |
CL&P | FERC ROE Complaints |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers |  |  |  |  |  |  |  |  |  |  | 21.4 | 21.4 |  |  |
CL&P | Deferred Pre-Staging And Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | 134.0 |  |  |  |
CL&P | Deferred Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | 1,120.0 |  |  |  |
CL&P | Deferred Storm Restoration Costs | Tropical Storm Isaias |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | 232.0 |  |  |  |
CL&P | Catastrophic Storm Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | 173.0 |  |  | $ 634.0 |
CL&P | Goodwill-related |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  | 0.0 |  |  |  | 0.0 | 0.0 |  |  |
NSTAR Electric |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  | 21.9 |  |  |  | 41.1 | 21.9 |  |  |
Deferred costs |  |  |  |  |  |  | 7.3 |  |  |  | $ 24.4 | $ 7.3 |  |  |
Historical average term of capital additions (in years) |  | 5 years |  |  |  |  |  |  |  |  |  | 5 years |  |  |
Rate increase approved |  |  |  |  |  |  |  | $ 104.9 |  |  |  |  |  |  |
Storms in a calendar year | storm |  |  |  |  |  |  |  |  |  |  | 8 |  |  |  |
Regulatory assets |  |  |  |  |  |  | 1,957.9 |  |  |  | $ 2,107.1 | $ 1,957.9 |  |  |
NSTAR Electric | FERC ROE Complaints |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers |  |  |  |  |  |  |  |  |  |  | 14.6 | 14.6 |  |  |
NSTAR Electric | Deferred Pre-Staging And Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | 85.0 |  |  |  |
NSTAR Electric | Deferred Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  |  |  |  |  |  |  |  | 551.0 |  |  |  |
NSTAR Electric | Goodwill-related |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  | 226.7 |  |  |  | 212.3 | $ 226.7 |  |  |
NSTAR Electric | Inflation Based Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  | $ 35.3 |  |  |  |  |  | 50.6 |  |  |  |  |  |  |
NSTAR Electric | K-Bar Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  |  |  |  |  |  |  | $ 54.3 |  |  |  |  |  |  |
NSTAR Electric | Base Rate Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  | 55.8 |  |  |  |  |  |  |  |  |  |  |  |  |
NSTAR Electric | Capital Additions Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  | $ 20.5 |  |  |  |  |  |  |  |  |  |  |  |  |
NSTAR Gas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  |  |  | $ 12.7 |  |  |  |  | $ 25.4 |  |  |  |  |  |
NSTAR Gas | Base Rate Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  |  |  |  |  |  |  |  | $ 15.5 |  |  |  |  |  |
Yankee Gas |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase (decrease) requested |  |  |  |  |  |  |  |  |  |  | 209.0 |  |  |  |
Yankee Gas | Base Rate Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase (decrease) requested |  |  |  |  |  |  |  |  |  |  | 274.0 |  |  |  |
Yankee Gas | Offsetting Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase (decrease) requested |  |  |  |  |  |  |  |  |  |  | 65.0 |  |  |  |
Yankee Gas | Rate Credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase (decrease) requested |  |  |  |  |  |  |  |  |  |  | $ 37.4 |  |  |  |
EGMA |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  |  | $ 76.8 |  |  |  |  |  |  |  |  |  |  |  |
EGMA | GSEP Investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  |  | 8.8 |  |  |  | 34.0 |  |  |  |  |  |  |  |
EGMA | Base Rate Adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  |  | $ 85.6 |  |  |  | $ 147.8 |  |  |  |  |  |  |  |
EGMA | Base Rate Adjustment | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, return on rate base, remaining revenue requirement | $ 62.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate Reduction Bonds | PSNH |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of securitized rate reduction bonds issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 635.7

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