SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) customer in Millions			3 Months Ended				6 Months Ended	9 Months Ended
	Sep. 23, 2023 USD ($)	May 25, 2023 USD ($) a project mile	Sep. 30, 2023 USD ($) project	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) project	Sep. 30, 2023 USD ($) regulatedUtility project customer	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric, natural gas and water customers | customer								4.4
Allowance for uncollectible accounts for late fees and other receivable amounts			$ 542,700,000	$ 532,400,000		$ 457,600,000	$ 542,700,000	$ 542,700,000	$ 457,600,000	$ 486,300,000	$ 457,800,000	$ 417,400,000
Number of offshore wind projects | project		3	3				3	3
Payments for investment								$ 887,365,000	617,588,000
Impairment charge			$ 0	401,000,000		0		401,000,000	0
After-tax impairment				331,000,000
Valuation allowance				$ 40,000,000
Percent realized of investment tax credit				0.10
Expected value of investment tax credit				$ 400,000,000
Energy Relief Fund - Merrimack Valley Programs
Summary of Accounting Policies [Line Items]
Restricted cash, current			20,000,000				$ 20,000,000	20,000,000		20,000,000
Restricted cash, noncurrent			14,300,000				14,300,000	14,300,000		15,900,000
COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			50,900,000				50,900,000	50,900,000		50,900,000
CL&P
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			276,300,000	262,800,000		198,600,000	276,300,000	$ 276,300,000	198,600,000	225,300,000	192,700,000	181,300,000
Period of accounts receivable recoverable under financial or medical duress								180 days
CL&P | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			16,000,000				16,000,000	$ 16,000,000		16,000,000
NSTAR Electric
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			100,800,000	97,800,000		99,500,000	100,800,000	100,800,000	99,500,000	95,000,000.0	100,500,000	97,000,000.0
NSTAR Electric | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			4,100,000				4,100,000	4,100,000		4,100,000
PSNH
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			12,400,000	12,700,000		$ 28,800,000	12,400,000	12,400,000	$ 28,800,000	29,200,000	$ 27,300,000	$ 24,300,000
PSNH | COVID-19
Summary of Accounting Policies [Line Items]
Allowance for uncollectible accounts for late fees and other receivable amounts			$ 0				$ 0	$ 0		$ 0
Yankee Gas
Summary of Accounting Policies [Line Items]
Period of accounts receivable recoverable under financial or medical duress								90 days
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | regulatedUtility								12
Offshore Wind Business
Summary of Accounting Policies [Line Items]
Ownership interest (percent)		50.00%	50.00%				50.00%	50.00%
North East Offshore
Summary of Accounting Policies [Line Items]
Ownership interest (percent)			50.00%				50.00%	50.00%
Renewable Energy Investment Fund
Summary of Accounting Policies [Line Items]
Ownership interest (percent)			90.00%				90.00%	90.00%
Proceeds from sale of equity method investments				$ 23,600,000	$ 123,400,000			$ 147,000,000
Renewable Energy Investment Fund | Equity Method Investee
Summary of Accounting Policies [Line Items]
Charitable contributions					$ 20,000,000
Uncommitted Lease Area
Summary of Accounting Policies [Line Items]
Ownership interest (percent)		50.00%
Number of miles off south coast of Massachusetts | mile		25
Uncommitted area of developable acres (in acres) | a		175,000
Payments for investment		$ 625,000,000
Tax Equity Capital Contribution Agreement
Summary of Accounting Policies [Line Items]
Payments for investment	$ 528,000,000
South Fork Wind Holdings, LLC Class A shares
Summary of Accounting Policies [Line Items]
Ownership interest (percent)			100.00%				100.00%	100.00%
Sunrise Wind LLC
Summary of Accounting Policies [Line Items]
Ownership interest (percent)			50.00%				50.00%	50.00%
Expected cash flows from sale of investment							$ 450,000,000