SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands, customer in Millions			1 Months Ended	3 Months Ended
	Feb. 12, 2024 USD ($)	Sep. 07, 2023 USD ($) a mile	Sep. 30, 2023 USD ($)	Mar. 31, 2024 USD ($) project regulatedUtility customer turbine	Mar. 31, 2023 USD ($)	Jan. 24, 2024	Dec. 31, 2023 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric, natural gas and water customers | customer				4.4
Number of offshore wind projects | project				3
Number of tax equity projects | project				1
Payments for investment				$ 307,433	$ 87,845
Number of turbines | turbine				6
Percent realized of investment tax credit	0.10
Expected value of investment tax credit	$ 170,000
Energy Relief Fund - Merrimack Valley Programs
Summary of Accounting Policies [Line Items]
Restricted cash, current				$ 20,000			$ 20,000
Restricted cash, noncurrent				$ 13,300			$ 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%
Offshore Wind Business
Summary of Accounting Policies [Line Items]
Ownership interest (percent)				50.00%
Northeast Offshore LLC And South Fork Class B Member, LLC
Summary of Accounting Policies [Line Items]
Ownership interest (percent)				50.00%
Number of offshore wind projects | project				3
South Fork Wind Holdings, LLC Class A shares
Summary of Accounting Policies [Line Items]
Ownership interest (percent)			100.00%	100.00%
Expected value of investment tax credit			$ 400,000	$ 193,000
Expected future cash flows of turbines			100,000
Increase (decrease) in taxes payable				(54,000)
Increase (decrease) in deferred tax assets				139,000
Deferred tax assets, tax equity method investments				$ 37,000
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%
Sunrise Wind LLC | Orsted
Summary of Accounting Policies [Line Items]
Equity method investment, percent of purchase price paid at closing (in percent)						0.50
Equity method investment, percent of remaining purchase price paid (in percent)						0.50
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
Cost sharing agreement, threshold amount	$ 240,000
Renewable Energy Investment Fund
Summary of Accounting Policies [Line Items]
Proceeds from sale of equity method investments					123,400
Renewable Energy Investment Fund | Equity Method Investee
Summary of Accounting Policies [Line Items]
Charitable contributions					$ 20,000
Orsted
Summary of Accounting Policies [Line Items]
Ownership interest (percent)						50.00%
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | regulatedUtility				12