SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Restricted Cash and Cash Equivalent Item [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	$ 259,340	$ 26,656
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	366,728	127,308	$ 202,801	$ 166,418
Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	85,500	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	10,300	10,000
Other Long-Term Assets
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	5,700	9,000
CL&P
Restricted Cash and Cash Equivalent Item [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	152,400	1,100
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	153,434	2,109	11,117	12,243
CL&P | Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	1,000	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	18,600	900
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	47,867	9,023	15,968	22,785
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	29,300	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	1,800	1,400
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	24,648	37,243	$ 26,693	$ 35,004
PSNH | Special Deposits
Restricted Cash and Cash Equivalent Item [Line Items]
Restricted cash	19,600	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