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