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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024	Dec. 31, 2023
Restricted Cash and Cash Equivalent Item [Line Items]
Cash and Cash Equivalents	$ 343,693	$ 26,656
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	437,786	127,308	$ 132,633	$ 166,418
Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	67,600	75,800
Assets Held for Sale, Current
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	5,900	5,800
Marketable Securities
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	11,700	10,000
Other Long-Term Assets
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	8,900	9,000
CL&P
Restricted Cash and Cash Equivalent Item [Line Items]
Cash and Cash Equivalents	99,200	1,100
Cash and Cash Equivalents as reported on the Balance Sheets	99,230	1,093
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	100,269	2,109	1,068	12,243
CL&P \| Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	1,100	1,000
CL&P \| Assets Held for Sale, Current
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
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 and Cash Equivalents as reported on the Balance Sheets	4,376	911
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	6,599	9,023	4,092	22,785
NSTAR Electric \| Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	2,200	8,100
NSTAR Electric \| Assets Held for Sale, Current
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
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 and Cash Equivalents as reported on the Balance Sheets	88	1,431
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	34,637	37,243	$ 34,081	$ 35,004
PSNH \| Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	31,300	32,700
PSNH \| Assets Held for Sale, Current
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	0	0
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	$ 3,100