SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2025 USD ($) customer regulatedUtility	Dec. 31, 2024 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric, natural gas and water customers | customer	4,600,000
Energy Relief Fund - Merrimack Valley Programs
Summary of Accounting Policies [Line Items]
Restricted cash, current	$ 20.0	$ 20.0
Restricted cash, noncurrent	$ 2.5	$ 5.9
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
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | regulatedUtility	12