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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2025
USD ($)
regulatedUtility
customer
Dec. 31, 2024
USD ($)
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 $ 6.1 $ 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