SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions					1 Months Ended	3 Months Ended				6 Months Ended
	Jul. 09, 2024 USD ($)	Apr. 18, 2024 USD ($)	Feb. 13, 2024 USD ($)	Sep. 07, 2023 USD ($) a mile	Sep. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) project	Dec. 31, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Jun. 30, 2024 USD ($) project regulatedUtility customer turbine	Jun. 30, 2023 USD ($)
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
Number of tax equity projects | project						1				1
Payments for investment										$ 729,354	$ 390,002
Number of turbines | turbine										12
Percent realized of investment tax credit			0.10
Expected value of investment tax credit			$ 170,000
Impairment of Offshore Wind Investment						$ 0	$ 1,770,000	$ 401,000		$ 0	401,000
After-tax impairment							1,620,000	331,000
Energy Relief Fund - Merrimack Valley Programs
Summary of Accounting Policies [Line Items]
Restricted cash, current						20,000	20,000			20,000
Restricted cash, noncurrent						$ 13,300	$ 14,100			$ 13,300
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%
Offshore Wind Business
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%
Number of offshore wind projects | project						3				3
South Fork Wind Holdings, LLC Class A shares
Summary of Accounting Policies [Line Items]
Ownership interest (percent)					100.00%	100.00%				100.00%
Expected value of investment tax credit										$ 385,000
Increase (decrease) in taxes payable										(54,000)
Increase in deferred tax assets										$ 331,000
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]
Proceeds from sale of equity method investments		$ 230,000
Sunrise Wind, LLC | Subsequent Event
Summary of Accounting Policies [Line Items]
Proceeds from sale of equity method investments	$ 118,000
Sunrise Wind, LLC | Orsted | Subsequent Event
Summary of Accounting Policies [Line Items]
Equity method investment, percent of remaining purchase price paid (in percent)	34,000,000
Orsted
Summary of Accounting Policies [Line Items]
Ownership interest (percent)		50.00%
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
Percent of investment tax credit			40.00%
Cost sharing agreement, threshold amount			$ 240,000
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