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REGULATORY ACCOUNTING - Narrative (Details) |  |  | 4 Months Ended | 12 Months Ended |  |  |  |  |  |
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Jan. 01, 2023 USD ($) | Nov. 01, 2022 USD ($) | Apr. 30, 2023 USD ($) | Dec. 31, 2022 USD ($) storm | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Mar. 31, 2022 USD ($) | Oct. 01, 2021 USD ($) | May 06, 2021 USD ($) | Mar. 31, 2021 USD ($) | Dec. 31, 2019 USD ($) | May 31, 2018 USD ($)
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | $ 210,800,000 | $ 252,500,000 |  |  |  |  |  |  |
Deferred costs related to uncollectible expense |  |  |  | 29,800,000 | 33,000,000 |  |  |  |  |  |  |
Regulatory Liabilities |  |  |  | 890,786,000 | 602,432,000 |  |  |  |  |  |  |
Gain (loss) related to litigation settlement |  |  |  | 72,041,000 | (81,274,000) | $ 0 |  |  |  |  |  |
Regulatory assets |  |  |  | 5,578,300,000 | 5,715,800,000 |  |  |  |  |  |  |
Increase (decrease) in regulatory assets |  |  |  | 205,294,000 | 314,211,000 | 516,411,000 |  |  |  |  |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows |  |  |  | 521,752,000 | 221,008,000 | 264,950,000 |  |  |  |  | $ 117,063,000 |
FERC ROE Complaints |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers |  |  |  | 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 |  |  |  | 399,000,000 |  |  |  |  |  |  |  |
Deferred Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | $ 1,400,000,000 |  |  |  |  |  |  |  |
Goodwill-related |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Recovery period (in years) |  |  |  | 40 years |  |  |  |  |  |  |  |
Remaining amortization (in years) |  |  |  | 17 years |  |  |  |  |  |  |  |
Regulatory assets |  |  |  | $ 281,000,000.0 | 297,800,000 |  |  |  |  |  |  |
Storm Costs, Net |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  | 1,379,100,000 | 1,102,700,000 |  |  |  |  |  |  |
PSNH |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | 1,000,000 | 3,400,000 |  |  |  |  |  |  |
Equity return not recorded on the balance sheet |  |  |  | 4,100,000 | 5,000,000 |  |  |  |  |  |  |
Regulatory Liabilities |  |  |  | 161,963,000 | 120,176,000 |  |  |  |  |  |  |
Regulatory assets |  |  |  | 696,200,000 | 786,400,000 |  |  |  |  |  |  |
Increase (decrease) in regulatory assets |  |  |  | (53,181,000) | (32,587,000) | 45,830,000 |  |  |  |  |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows |  |  |  | 36,812,000 | 35,126,000 | 39,555,000 |  |  |  |  | 36,688,000 |
PSNH | FERC ROE Complaints |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers |  |  |  | 3,100,000 |  |  |  |  |  |  |  |
PSNH | Deferred Pre-Staging And Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | 55,000,000 |  |  |  |  |  |  |  |
PSNH | Deferred Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | 98,000,000 |  |  |  |  |  |  |  |
PSNH | Goodwill-related |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  | 0 | 0 |  |  |  |  |  |  |
PSNH | Storm Costs, Net |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  | 95,400,000 | 65,800,000 |  |  |  |  |  |  |
CL&P |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | 135,900,000 | 114,900,000 |  |  |  |  |  |  |
Deferred costs related to uncollectible expense |  |  |  | 11,800,000 | 18,000,000 |  |  |  |  |  |  |
Regulatory Liabilities |  |  |  | 336,048,000 | 266,489,000 |  |  |  |  |  |  |
Gain (loss) related to litigation settlement |  |  |  | 72,041,000 | (81,274,000) | 0 |  |  |  |  |  |
Regulatory assets |  |  |  | 1,907,800,000 | 2,084,800,000 |  |  |  |  |  |  |
Increase (decrease) in regulatory assets |  |  |  | 144,793,000 | 152,775,000 | 279,941,000 |  |  |  |  |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows |  |  |  | 20,327,000 | 74,788,000 | 99,809,000 |  |  |  |  | 4,971,000 |
CL&P | Energy Assistance Program |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Restricted cash |  |  |  | 10,000,000 |  |  |  |  |  |  |  |
CL&P | Customer Assistance Fund |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows |  |  |  |  |  |  | $ 10,000,000 |  |  |  |  |
CL&P | Forecast |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Average monthly bill |  |  | $ 10 |  |  |  |  |  |  |  |  |
Public utilities, approved rate increase (decrease), net revenues generated |  |  | $ 90,000,000 |  |  |  |  |  |  |  |  |
CL&P | FERC ROE Complaints |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers |  |  |  | 21,400,000 |  |  |  |  |  |  |  |
CL&P | Unfavorable Regulatory Action |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Loss contingency, estimate of possible earnings impact, non-compliance with performance standards |  |  |  |  |  |  |  |  | $ 28,400,000 | $ 28,400,000 |  |
Period credits are issued to customers (in years) |  |  |  |  |  |  |  |  | 1 year |  |  |
Regulatory Liabilities |  |  |  | 75,000,000 | 75,000,000 |  |  |  |  |  |  |
Customer credits |  |  |  |  |  |  |  | $ 65,000,000 |  |  |  |
Customer assistance initiatives |  |  |  |  |  |  |  | $ 10,000,000 |  |  |  |
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 |  |  |  | 163,000,000 |  |  |  |  |  |  |  |
CL&P | Deferred Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | 807,000,000 |  |  |  |  |  |  |  |
CL&P | Deferred Storm Restoration Costs | Tropical Storm Isaias |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | 235,000,000 |  |  |  |  |  |  |  |
CL&P | Goodwill-related |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  | 0 | 0 |  |  |  |  |  |  |
CL&P | Storm Costs, Net |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  | 799,300,000 | 695,600,000 |  |  |  |  |  |  |
NSTAR Electric |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | 19,800,000 | 85,000,000 |  |  |  |  |  |  |
Deferred costs related to uncollectible expense |  |  |  | 2,200,000 | 6,100,000 |  |  |  |  |  |  |
Regulatory Liabilities |  |  |  | $ 373,221,000 | 228,248,000 |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |
Authorized regulatory ROE (in percent) |  |  |  | 9.80% |  |  |  |  |  |  |  |
Authorized common equity ratio (in percent) |  |  |  | 53.20% |  |  |  |  |  |  |  |
Regulatory assets |  |  |  | $ 1,714,400,000 | 1,579,200,000 |  |  |  |  |  |  |
Increase (decrease) in regulatory assets |  |  |  | $ 88,220,000 | 79,075,000 | 186,081,000 |  |  |  |  |  |
Storms in a calendar year | storm |  |  |  | 8 |  |  |  |  |  |  |  |
Storms recovered at base rate | storm |  |  |  | 6 |  |  |  |  |  |  |  |
Number of additional storms | storm |  |  |  | 2 |  |  |  |  |  |  |  |
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows |  |  |  | $ 345,293,000 | 18,179,000 | $ 17,410,000 |  |  |  |  | $ 6,312,000 |
NSTAR Electric | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved | $ 64,000,000 |  |  |  |  |  |  |  |  |  |  |
NSTAR Electric | FERC ROE Complaints |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers |  |  |  | 14,600,000 |  |  |  |  |  |  |  |
NSTAR Electric | Deferred Pre-Staging And Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | 181,000,000 |  |  |  |  |  |  |  |
NSTAR Electric | Deferred Storm Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount of regulatory costs not yet approved |  |  |  | 492,000,000 |  |  |  |  |  |  |  |
NSTAR Electric | Goodwill-related |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  | 241,200,000 | 255,700,000 |  |  |  |  |  |  |
NSTAR Electric | Storm Costs, Net |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  | 484,400,000 | $ 341,300,000 |  |  |  |  |  |  |
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 | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
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 | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
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 | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets | $ 8,300,000 |  |  |  |  |  |  |  |  |  |  |
Recovery period of regulatory asset (in years) | 5 years |  |  |  |  |  |  |  |  |  |  |
NSTAR Gas |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Rate increase approved |  | $ 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 |  |  |  |  |  |  |  |  |  |
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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