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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions |  |  |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 09, 2024 USD ($) | Apr. 18, 2024 USD ($) | Feb. 13, 2024 USD ($) | Sep. 23, 2023 USD ($) | Sep. 07, 2023 USD ($) a mile | Sep. 30, 2024 USD ($) project | Sep. 30, 2023 USD ($) | Jun. 30, 2023 USD ($) | Mar. 31, 2023 USD ($) | Sep. 30, 2024 USD ($) project regulatedUtility customer | Sep. 30, 2023 USD ($) | Dec. 31, 2023 USD ($) | Jan. 24, 2024
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of electric, natural gas and water customers | customer |  |  |  |  |  |  |  |  |  | 4.4 |  |  |
Number of offshore wind projects | project |  |  |  |  |  | 3 |  |  |  | 3 |  |  |
Payments for investment |  |  |  |  |  |  |  |  |  | $ 929,651 | $ 887,365 |  |
Percent realized of investment tax credit |  |  | 0.10 |  |  |  |  |  |  |  |  |  |
Expected value of investment tax credit |  |  | $ 170,000 |  |  |  |  |  |  |  |  |  |
Losses on Offshore Wind Investments |  |  |  |  |  | $ 464,019 | $ 0 | $ 401,000 |  | 464,019 | $ 401,000 | $ 1,770,000 |
After-tax impairment |  |  |  |  |  |  |  | 331,000 |  |  |  | 1,620,000 |
Energy Relief Fund - Merrimack Valley Programs |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted cash, current |  |  |  |  |  | 20,000 |  |  |  | 20,000 |  | 20,000 |
Restricted cash, noncurrent |  |  |  |  |  | $ 13,800 |  |  |  | $ 13,800 |  | $ 14,100 |
CL&P |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Period of accounts receivable recoverable under financial or medical duress |  |  |  |  |  |  |  |  |  | 180 days |  |  |
Yankee Gas |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Period of accounts receivable recoverable under financial or medical duress |  |  |  |  |  |  |  |  |  | 90 days |  |  |
Tax Equity |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) |  |  |  |  |  | 50.00% |  |  |  | 50.00% |  |  |
Northeast Offshore LLC And South Fork Class B Member, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) |  |  |  |  |  | 50.00% |  |  |  | 50.00% |  |  |
South Fork Wind Holdings, LLC Class A shares |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) |  |  |  |  |  | 100.00% | 100.00% |  |  | 100.00% | 100.00% |  |
Expected value of investment tax credit |  |  |  |  |  |  |  |  |  | $ 459,000 |  |  |
Decrease in taxes payable |  |  |  |  |  |  |  |  |  | 54,000 |  |  |
Increase in deferred tax assets |  |  |  |  |  |  |  |  |  | 405,000 |  |  |
Uncommitted Lease Area |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) |  |  |  |  | 50.00% |  |  |  |  |  |  |  |
Uncommitted area of developable acres (in acres) | a |  |  |  |  | 175,000 |  |  |  |  |  |  |  |
Number of miles off south coast of Massachusetts | mile |  |  |  |  | 25 |  |  |  |  |  |  |  |
Payments for investment |  |  |  |  | $ 625,000 |  |  |  |  |  |  |  |
Tax Equity Capital Contribution Agreement |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payments for investment |  |  |  | $ 528,000 |  |  |  |  |  |  |  |  |
Sunrise Wind, LLC |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of equity method investments | $ 118,000 | $ 230,000 |  |  |  | $ 377,000 |  |  |  |  |  |  |
Sunrise Wind, LLC | Orsted |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) | 50.00% |  |  |  |  |  |  |  |  |  |  |  | 50.00%
Proceeds from sale of equity method investments | $ 152,000 |  |  |  |  |  |  |  |  |  |  |  |
Sunrise Wind, LLC | Orsted |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, remaining purchase price paid | $ 34,000 |  |  |  |  |  |  |  |  |  |  |  |
South Fork Wind And Revolution Wind |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (percent) |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
Proceeds from sale of equity method investments |  |  | $ 1,100,000 |  |  | 745,000 |  |  |  |  |  |  |
Adjustments from sale of equity method investment |  |  |  |  |  | 375,000 |  |  |  |  |  |  |
Adjustment from sale of equity method investment, capital expenses |  |  |  |  |  | 150,000 |  |  |  |  |  |  |
Adjustment from sale of equity method investment, non-construction costs |  |  |  |  |  | 225,000 |  |  |  |  |  |  |
Percent of investment tax credit |  |  | 40.00% |  |  |  |  |  |  |  |  |  |
Cost sharing agreement, threshold amount |  |  | $ 240,000 |  |  |  |  |  |  |  |  |  |
Equity method investment, realized gain (loss) |  |  |  |  |  | (524,000) |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  |  |  | 350,000 |  |  |  | 350,000 |  |  |
Equity method investment, post-closing sale costs |  |  |  |  |  | 326,000 |  |  |  | 326,000 |  |  |
Effective income tax rate reconciliation, change in enacted tax rate, amount |  |  |  |  |  | 60,000 |  |  |  |  |  |  |
Equity method investment, liability obligations |  |  |  |  |  | $ 365,000 |  |  |  | $ 365,000 |  |  |
Renewable Energy Investment Fund |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of equity method investments |  |  |  |  |  |  |  | $ 23,600 | $ 123,400 |  | $ 147,000 |  |
Renewable Energy Investment Fund | Equity Method Investee |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Charitable contributions |  |  |  |  |  |  |  |  | $ 20,000 |  |  |  |
Connecticut, Massachusetts and New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of regulated utilities | regulatedUtility |  |  |  |  |  |  |  |  |  | 12 |  |  |

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