SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents	$ 111,362	$ 26,656
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	200,618	127,308	$ 360,396	$ 166,418
Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	64,600	75,800
Assets Held for Sale, Current
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	5,800	5,800
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	9,500	10,000
Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	9,300	9,000
CL&P
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents	67,700	1,100
Cash and Cash Equivalents as reported on the Balance Sheets	67,690	1,093
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	68,717	2,109	31,055	12,243
CL&P | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	1,000	1,000
CL&P | Assets Held for Sale, Current
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
CL&P | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
CL&P | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
NSTAR Electric
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	2,907	911
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	11,103	9,023	36,503	22,785
NSTAR Electric | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	8,200	8,100
NSTAR Electric | Assets Held for Sale, Current
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
NSTAR Electric | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
NSTAR Electric | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
PSNH
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents as reported on the Balance Sheets	4,397	1,431
Cash, Cash Equivalents and Restricted Cash as reported on the Statements of Cash Flows	30,141	37,243	$ 34,311	$ 35,004
PSNH | Special Deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	22,500	32,700
PSNH | Assets Held for Sale, Current
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
PSNH | Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	0	0
PSNH | Other Long-Term Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 3,200	$ 3,100