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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025	Jun. 30, 2025	Dec. 31, 2024
Restricted Cash and Cash Equivalent Item [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 1,819,319	$ 135,351
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	1,894,051	246,245	$ 437,786	$ 127,308
Balance Sheet Location [Axis]: us-gaap:MarketableSecurities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	11,900	14,000
Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	3,200	3,200
Balance Sheet Location [Axis]: us-gaap:PrepaidExpenseAndOtherAssetsCurrent
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	59,700	93,600
CL&P
Restricted Cash and Cash Equivalent Item [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	500	87,600
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	1,608	88,676	100,269	2,109
CL&P \| Balance Sheet Location [Axis]: us-gaap:MarketableSecurities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
CL&P \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
CL&P \| Balance Sheet Location [Axis]: us-gaap:PrepaidExpenseAndOtherAssetsCurrent
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	1,100	1,100
NSTAR Electric
Restricted Cash and Cash Equivalent Item [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	15,400	8,300
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	16,611	25,944	6,599	9,023
NSTAR Electric \| Balance Sheet Location [Axis]: us-gaap:MarketableSecurities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
NSTAR Electric \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
NSTAR Electric \| Balance Sheet Location [Axis]: us-gaap:PrepaidExpenseAndOtherAssetsCurrent
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	1,200	17,600
PSNH
Restricted Cash and Cash Equivalent Item [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	56,074	13,665
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	93,670	55,186	$ 34,637	$ 37,243
PSNH \| Balance Sheet Location [Axis]: us-gaap:MarketableSecurities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
PSNH \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	3,200	3,200
PSNH \| Balance Sheet Location [Axis]: us-gaap:PrepaidExpenseAndOtherAssetsCurrent
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	$ 34,400	$ 38,300