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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Restricted Cash and Cash Equivalent Item [Line Items]
Cash as reported on the Balance Sheets	$ 270,200	$ 135,400
Cash and Restricted Cash as reported on the Statements of Cash Flows	337,791	246,245	$ 200,618	$ 127,308
Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	56,200	93,600
Marketable Securities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	8,200	14,000
Other Long-Term Assets
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	3,200	3,200
CL&P
Restricted Cash and Cash Equivalent Item [Line Items]
Cash as reported on the Balance Sheets	13,000	87,600
Cash and Restricted Cash as reported on the Statements of Cash Flows	14,117	88,676	68,717	2,109
CL&P \| Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	1,100	1,100
CL&P \| Marketable Securities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
CL&P \| Other Long-Term Assets
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
NSTAR Electric
Restricted Cash and Cash Equivalent Item [Line Items]
Cash as reported on the Balance Sheets	190,800	8,300
Cash and Restricted Cash as reported on the Statements of Cash Flows	190,821	25,944	11,103	9,023
NSTAR Electric \| Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	17,600
NSTAR Electric \| Marketable Securities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
NSTAR Electric \| Other Long-Term Assets
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
PSNH
Restricted Cash and Cash Equivalent Item [Line Items]
Cash as reported on the Balance Sheets	3,100	13,700
Cash and Restricted Cash as reported on the Statements of Cash Flows	28,549	55,186	$ 30,141	$ 37,243
PSNH \| Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	22,200	38,300
PSNH \| Marketable Securities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
PSNH \| Other Long-Term Assets
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	$ 3,200	$ 3,200